Why did Sakata Gintoki need to go to the future? In Gintama the Movie: Be Forever Yorozuya (Yorozuya yo Eien Nare), why did Gin need to go to the future first? 

He was infected by the White Plague during the Joui war, so why did the future Gin call him to the future to kill the plagued future Gin instead of just sending him to the past and preventing the plague from spreading in the first place?
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.