The most common reason that swimmers will use yoga is to become more flexible.
I tend to stray away from the word flexibility, I feel that mobility is much more appropriate. Why I am specific about the language is that in an attempt to become more flexible many sacrifice stability to increase range of motion. By doing this joints can be compromised.
When focusing on mobility in a yoga practice it will increase range of motion and lengthen muscle tissue while stabilizing a joint forming greater integrity and strength.