this was short and sweet but here is my idea
 
after that nightmare griffin knew he can’t make amends with the ponies he hurt (if you read the original book you know) mainly because
 
1: they think he’s dead  
2: he will put himself in danger if they found out he is alive  
and 3: he will put his friend in danger
 
but he can help make amends with one pony even though he’s dead, his father he wants to repair his father’s reputation so he (and Matthew) travailed to the town which was a town he knew perhaps as a child, a summery said in a way the town looking all desolate (i guess) symbioses griffin’s former self being replace by something of less value (his experiment most likely) but now he came back and the town now looks better then ever.
 
the reason for this is to symbolize griffin’s former self coming back and better then ever which means he cured in a way from his bad mentality because of the stress he experience four years straight