Why did Sakata Gintoki need to go to the future?
Maybe because of the potential consequences of changing the past, Gintoki may not have met Shinpachi, Kagura and others at all.

Moreover, if you believe in the "parallel world" interpretation of time traveling, it can be said that the past cannot be changed, but only forked into alternative futures. 

Thus maybe going to the future is the best choice here.