If you use a goon style deck, a velocity style deck or a postless deck, the coily tool is invaluable.
For postless decks, if the coil legs are too long it affects the final height of your coil, so fitting a coil can involve lots of trial and error, trying it, trimming a bit, trying it again, trimming a bit more etc etc.
Once you get it right once, when you remove the coil stick it in the coily and find what length you need for next time. It makes it a doddle to trim legs to exactly the length required.
With goon or velocity style decks, where dual coils are fitted back to back, it is really awkward to trim the second coils legs after mounting them, as the first coils legs are in the way.
This handy device allows you to pre trim your legs easily to the correct length and mount the coils without all the faff.
I wouldn't be without mine either.
But if you don't use postless or dual back to back decks, you won't see the advantages.