Like a few things in Legend, the answer is legacy. Legend is based on Dungeons and Dragons 3.5, despite the many massive changes that have been made, and some things exist for no other reason than to make 3.5 players comfortable. In this case, there was also a desire on the part of some portions of the Legend to show that Legend could handle traditional spellcasting well, despite its unusual advancement system.
Historically, there was a point when Legend did not have any spellcasting tracks; the original magical class was Sage, and other spellcasters were going to follow the same pattern: each circle would give some Spell-like Abilities, or a choice of Spell-like Abilities, which would be per-round, per-encounter, or per-scene as appropriate to their power and utility.
However, quite early on, before the 2010 alpha even, there were a lot of requests for a “proper” spellcasting track, complete with Vancian(-ish, in deference to the valid points made in the comments) mechanics. So the Shaman and Tactician spellcasting tracks were added in response to player demand.
This all took place very early in Legend’s lifetime. I first became involved in Legend several months prior to the November 2012 beta release, and the spellcasting tracks were well-established by that time; I heard this history from the original creative director, Jake Kurzer. The original May 2010 alpha did not have the Shaman and Tactician tracks, though it does refer to the fact that it is a “non-casting classes only” release, despite including the Sage, so even then there was acknowledgement that spellcasting would be inevitable.