There is the wolverine setup, which has the bases and the arms that extend out, one with a platform, the other with sort of a pocket. The “jig” is the varigrind - easily confused.
There are quite a few grinds (shapes) for bowl gouges. There are far fewer for spindle gouges. In general jigs allow different set ups depending on the desired grind, and most importantly, make reproducing near exactly that same grind shape when resharpening.
A traditional bg grind can be done pretty easily by dropping the handle end of the gouge into the pocket, setting the angle by moving the pocket in/out, then rotating the tool using dwell time to get the desired bevel shape.
The long wing bg grinds (search Ellsworth) are completely different. I dont know of anyone today that does them without a jig, tho with enough practice It could be done. There is a side to side and tool rotation motion that is difficult to always reproduce by hand.
One popular bg grind done by hand on a platform is the 40/40 (search Stuart Batty 40/40). Its in between the traditional and long wing grinds. Since I have periods of months I dont turn, I prefer to use a jig for this grind (not exact but very close) so I dont have to retrain myself when I get back to it. Do a search for bowl gouge grinds and you will find all kinds of good and bad info. Someone here can probably link a good website/article.
There are several good jigs, including some diy, that work well. All are the same concept but vary in adjustability, set up, and resulting bevel geometry. Turners have various preferences for various reasons and applications.