Sleepy realization: Kits used to be about different ways of doing things within the class structure, like the thief’s swashbuckler in BG. For most of the others in 2e/BG, kits are about the purpose of said kit.
Introduced in 2e, kits started out as barely more than what the term implies: a load-out for a particular play style. Along with this, some cultural context might be included, or associating them with certain backgrounds, personalities, or alignments. As each class got its own handbook, rules tweaking crept in, where instead of the content being mostly about fluff and role-playing, you got the mechanical push/pull kits like most of the ones in BG: a few bonuses to class skills, a small selection of activated abilities, balanced by penalizing or removing inherent class benefits.