These spells are called "invocations", because they're calling upon a god or goddess (either one directly in the same dimension or in another one, with some confusing restrictions) to remotely supply the rest of the pattern and energy for the spell. Sometimes these invocations have direct costs, but more usually the deity is happy to do this for "free". It's not super well understood by anyone Buffy knows exactly why, but invocations tend to always be effects which align with a deity's domain, and casting a lot of them tends to quickly align both the caster's energy reserve and their mindset in general with that of the deity. Because of this invocations are regarded as a more dangerous type of spell to get used to spamming and they're the common culprit for spellcasters getting "addicted" and going off the deep end. On the other hand invocations give a lot more bang for your buck and are an easy way to get yourself into the groove of a certain type of magic.