To put him into the right place, namely Caemlyn, for ToM. You can't simply skip the whole travel and you have to let Mat think about his new status at one point. In my view, it was good to have dealt with this in TGS, so that the action can happen in ToM:

In Caemlyn Mat can start to build the cannons, prepare his trip to the Finn (maybe Birgitte can give additional hints? Mat might get the needed items to beat the Finn), maybe already fulfill Verin's orders, confront the gholam, leave Olver, the Band and the rest of his people in Elayne's care, etc.

Furthermore, I think the Hinderstap episode was a nice action scene and stresses how badly the DO affects ordinary people, while the Pattern tries to counter it. Besides that, Mat brought fun in an otherwise very dark book.