It's also worth noting that originally cheap hordes were still very useful back when "Chargers always strike first [barring magic items or special rules], everyone fights in one rank by default, casualties get removed from attacks back, and there is no Steadfast".
Specifically you tended to wield them in one of two fashions:
+ First, you gave them spears. Spears weren't amazing for a lot of non-High Elf and non-Lizardmen armies [a lot of the troops who could be armed with them would be WS2 to WS3, WS4 for rare exceptions like Druchii and Asrai; likewise most of them would be S3 without nice features such as Armor Piercing]. But what they did was effectively brace against light cavalry, beasts [such as dire wolves], units relying on charge bonuses to break [Marauders with Flails, who otherwise might as well have been Empire Swordsmen if they failed to break on the charge], or similarly low quality infantry [such as Skeletons & Zombies, Bretonnian Men At Arms, even things like lightly armored Orcs].
The solid base of three ranks + standard + in several cases [or at least ideally in the case of Undead] Outnumber meant that in terms of active combat resolution they started with a floor of five that could potentially increase with attacks back [see High Elves fighting in three ranks with their spears, or Saurus getting additional attacks from the front rank and being Heavy Infantry]. Likewise if the unit was relying on the charge to succeed, there was a risk calculus for what happens if they failed to break through [often times, getting mulched by disproportionate attacks back in later turns]. Any attached characters + Champion were intended to eat up any of their own front rank characters / Champions, because ignoring them meant guaranteed attacks back [and again these were often situations the attacker was banking on active combat resolution to Break] and winnowed the differences in Combat Resolution significantly.
+ The second use of cheap hordes was "Light armor, hand weapons + shields, maybe a War Banner if it's available". These bricks served all of one purpose and one purpose only: Being cheaper anvils than more heavy alternatives. Back in the 6E and 7E era high strength plus armor piercing was relatively limited, likewise attacks going out, so a unit with a 4+ save [or, better yet, a 4+ save and T4] could be irksome to body. And while they couldn't press with the attacks back like Spearmen [and didn't have the Steadfast or 6++ saves of 8E], that was never their goal. Their goal was to bring somebody's battle line to a halt. Force them to either need to hold back in the case of counter-charges, or maneuver to flank a unit that in the end probably only cost their opponent a good 200-250 points [I remember my 25 Orcs w/ Full Command and Shields cost something like 205 points] and so took disproportionate deployment and maneuvering to dislodge.
The exceptions tended to be, for points of easy comparison, things like either Night Goblins or 7E Common Goblins.
+ The Night Goblin style units were those that had something attached to make things even more miserable for their chargers. Night Goblins released Fanatics, who - depending on the edition - were anywhere from 3d6 to 6d6 S5 hits on the enemy if things went wrong. This was basically an insurance by the Night Goblins against either Heavy Cavalry [for whom even just 1d6 S5 No Save hits was potentially maneuver ruining] or similar bricks [since a good 2-4 d6 hits risked taking out a couple ranks, which could give the Night Goblins more starting Active Resolution to be overcome]. Other armies had this in the form of things like Skaven Weapon Teams, or Empire Detachments. Units that via special rules supplemented others: Handgunners might Stand and Shoot [risking the loss of a rank bonus or even - if particularly violent - causing a Panic check], the State Troops counter-charging, etcetera. Skaven would often go ham at these points because a lot of their stuff was particularly dangerous and vicious at close range [Warpfire Throwers, Gatling guns using more than 1D6, etc].
Point is they supplemented by softening the target one last time before they hit proper, hoping that the base unit's sheer size and cheapness [40 Night Goblins, with Command and two Fanatics, was approximately 160-200ish points depending on the edition; Clanrats not much different though probably only 30 big at that price range].
+ 7E Common Goblins are used as a reference due to being in the same line, but a more apt comparison might be Skaven or even Dark Elves. Specifically these units had the risk of hiding something in it that was a big Fuck You. In Common Goblins this was Sneaky Skulkers, who came out with something like 6-9 attacks that had Killing Blow on the first turn and, IIRC for 7E too [definitely 8E], ASF on the turn they're revealed. These Skulkers were basically Goblin stats so they were minimally useful for active combat resolution... but were goddamn terrifying for attached characters charging the unit because Killing Blow. Yeah, you have a nice hammer of 18 remaining Iron Breakers lead by a Thane. But do you really want to risk that Thane getting unlucky and eating shit? They're Goblins, there's a one-in-six chance they spend the next turn bickering harmlessly and you can put more munitions into them. And if they charge what are they going to do? They're Goblins.
For other armies the calculus might be a bit more different (Druchii and Skaven Assassins are no slouches, being high WS and in the Skaven's case S4 to boot before magic items or venoms on either], but being able to also attack basic troops just further made clear what their function was. Mind games. Did they take an Assassin? Can you be sure? Does that look like 2000 points of stuff in front of you? Would they have put it in with their anvil as a Fuck You surprise to anyone trying to break the flank, or did they hide it with their General inside the Stormvermin to make a deathstar?
tl;dr Before 8E there were a number of uses for low-stat mediocre equipment units between just existing and also special rules that often came with them or their army.