Jujitsu's beginning can be traced back a far as 22 BC as a type of wrestling. It wasn't until over a century later that and art was made from this wrestling. Jujitsu involved moves that were not based on the assumption that you were a big powerful person. Most of the moves can cause great pain and the breaking of bones with very little power applied. This is especially noticeable with pressure points.
Pressure points can cause great pain and very little pressure has to be applied.
This gives them two uses. This type of art was something new for Japan. Carrying bladed weapons was very common and successful unarmed defense against them was a great asset. Yoshimitsu was a General for the family Minamoto family and jujitsu was kept a secret inside that family for centuries. From 1333 to 1573 jujitsu was very big all over Japan.