In Aikido, the practice of ukemi, beyond the obvious fitness' reasons, has 2 reasons:
- allow the tori to perform techniques without restraint. Technically, the technique is as good as it unbalances uke. A good uke allows tori/shite to focus a bit more on the technique rather than the safety of his partner.
The second reason is less obvious and more interesting:
- uke learns to keep control when unbalanced. Overcoming instincts of keeping balance at all costs, he will be more and more able to keep calm when in compelling situations.
...the second reason is the big deal, even in dojo practice. Aikido is frustrating, and spending half of the time as uke means having to deal with the fear of falling for a lot time. Overcoming the fear, leads you to overcome the frustration and to the fullness of your practice. This is not just theory, it's an empiric concept: just look at the practice of good ukes versus bad ones.
All in all, that is the essence of budo: stepping out of the mat better than you were before stepping in: with no harms, without frustration, stronger.
Don't let anybody convince you that ukemi is just a show off.