actually it is also about the ranking for him: he has said this many times and he said it again last week. he said he would like to be #1 again.
so winning titles--large and small--will get him to where he wants to go. not saying he will get there definitely but that is his thinking anyway.
he is working hard on his fitness and his game. there is a reason for that.
It's my belief that it Federer's fitness that is letting him down in the big matches in the grand slams (alongside the Nadal match up issue). Take the Djokovic match in the US Open in 2011. Federer played out of his mind putting everything into the first 2 sets, a level of play he has to reach to beat Djokovic. But he took his foot off the pedal in sets 3 and 4, and Djokovic came back into the match, as Federer stopped moving as well. Then it was noticeable how Federer upped his level in the 5th set, working much harder to retrieve balls to stay in points. He should've won the match. But that's my point. His fitness is not good enough to compete against Djokovic or Nadal at the required level over 5 sets on slow courts. In fact Federer in al likelihood would struggle against Murray on slow courts as well over 5 sets (barring a Murray choke). Had Federer beaten Djokovic in 2011 US Open semi-final then I am certain Nadal would've beaten him in the final as that centre court looked very slow, and Federer just would not have been able to hang tough in the long rallies, something that is critical in beating Nadal as Djokovic has shown again and again.
As for winning 100 titles? All down to whether Federer wants too. As others have said, he could quite easily just play 20 ATP250 events every year and win them all for the next 5 years as the opposition just is nowhere good enough to beat him bar whomever reaches the final. But Federer won't, as he wants the big titles. The ones which are hard to win. They define greatness. The slams.