One of our kata has overhead twirl, pass behind the back and change hands and strike manouvres. Ushiro Dori, Daisha Mawashi & Tsuki.
Being able to move the bo from one side/part of the body to the other quickly and keep it from being grabbed and bringing it into a strike position is useful. Hence keeping it moving so the end cannot be grabbed.
Parts of other kata have two handed side to side twirling movements [Hachiji Mawashi - figure 8 movement] that are similar in effect to redondo used in FMA stick fighting. They serve to keep the opponent well away until you can line them up to strike them.
Shimabuku has a two handed overhead sweep/twirl targeting the head/neck which would be devastating if it hit....
If we look at Demura's book on the bo we find Ushiro Dori [back grab] and Daisha Mawashi [large circular movement] both of which incorporate single hand movements of the bo twirling around the body. And Harai Uke which is the 2 handed overhead twirl and downward block we see in many kata.
Here endeth the technical bit.
In my book any exercise that brings better handling of a weapon is usually a good thing. It makes the weapon more a part of the practioner and you learn to work with it on a more natural level.
So, go forth and play with your weapons until you know them better!