In most role-playing games, character progression is tied to adventuring, whether that is through the accumulation of experience points or the successful use of skills.
However, some d100 games (e.g. RuneQuest, Call of Cthulhu) have a secondary mechanism for character improvement, namely training in the downtime between adventures.
On the face of it, this seems plausible. Why shouldn’t study or practice be a route to improvement? However, equally, in the interests of an exciting game, training shouldn’t become the primary route for character development.