Probably the reason most similar to straight-up drama rehearsals, players simply need to learn what to do when. Whether it's learning basic stagecraft for newbies, learning introductory improv skills, understanding the rules, gimmicks and flow of stage games, students need to learn the mechanics of what they're performing on stage. For some games that require special roles - like playing the Bachelor/Bachelorette in Dating Game, or serving as the Host for a show - playing those games again and again will give those players a better sense of how to "lead" those games in their role.
The Good, The Bad and The Ugly
Troupe members need to know what their strengths and weaknesses are, and what they need to improve upon. Improv rehearsals focus on just that - targeting a specific skill for improvement. This one often falls to the coach of the troupe to decide what skills need to be practiced, and to give some guidance as to what games and exercises to play to develop them. Some advanced improvisers might be self-aware enough to know their own strengths and weaknesses, but often the guiding hand of a third party - not only a coach, but their fellow players in a supportive atmosphere - can bring about improvement.
Everyone's A (Constructive) Critic
One of the best tools during an improv rehearsal is the breakdown - after playing a scene or performing an exercise, everyone in the group (with the guidance of the coach) breaks down what worked and what didn't. During rehearsals, those troupe members who are the audience members for a game or exercise have the job of watching what goes well and what could be improved - both for the performers' benefits and their own. Newbies get a chance to see what "works" in a scene (or, sometimes more informatively, what doesn't), and then they need a chance to get up and do it. Of course, vets need the same...
Smells Like Team Spirit
The single biggest reason is to work with the group. Chemistry in a drama ensemble is often very important, but a lack of chemistry can be worked around. Chemistry in an improv troupe is vital - without it, scenes are almost impossible to do well. Rehearsal allows all members of the troupe to figure out how to play with each other - which pairings work well, what each member's strengths and weaknesses are, etc. A shared knowledge base is something that everyone in the ensemble can draw from in any scene - if they're practiced enough, the other ensemble members can often tap into that knowledge base at the same time and bring synergy to the scene. That only happens with practice and experience, though.
A good improv team rehearses frequently. A great improv team rehearses regularly and with a set goal...
Creative people need to be, and thrive when, challenged.
Don't underestimate the importance of rehearsal - regular, well-structured rehearsal - for the life and ability of your improv troupe.