Why would you need trails to build the docks.. Forests are widespread, even in Blackmouth.
The gravel is gotten from mining so you just build gravel trails extending outward from the mines.
It would b cool.
That was more of a joke post but I'll answer a bit more seriously. No citations because wow, it takes so much effort.
First off, I wasn't that clear (or even clear at all since I was purposefully obfuscating what I meant) but I was focusing on the Stallion Tribes and Western Wall.
The forests there are relatively new (besides the first planting by the Stallions long, long ago), so there isn't that much growth, especially if they want to grow the forest for defense instead of cutting it down. So they'll want to ship lumber from elsewhere which requires boats or lumber. Small boats probably aren't that great for lumber transport.
There's a lack of trails, groomed or gravel, to the area. To lay down gravel, you need gravel in the immediate area. This requires transport of gravel from the source, in this case, the mines. The mines are relatively deep into our territory and need to transport gravel all the way out to the Stallions to lay their trails.
We lack even basic trails to make it all the way there, so we might want to make Groomed Trails first to have a smooth-ish path to the Stallions. The initial trails will also help mark where to lay down the gravel since it would really suck if somebody laid out a gravel trail that went the long way around something.
We want the initial trails to make upgrading trails easier.
These New Trails are capped by Centralization which we have no way to efficiently dump. (Also, we're raising it through Enforce Justice.)
But hey, this all started with a weird idea of shipping gravel to the Stallions even if it's horribly inefficient compared to just transporting over the (non-existent) roads. It just made my previous post more fun to type out. I can't even defend this point.