Short answer: they are good for the platform holder and the platform and bad for 3rd-parties and consumers.
In more detail:
Obviously they enhance the value of a platform. They are mostly the reason people bought certain consoles in the past.
Also good for the platform: exclusives really can lean into special features of the platform. That was great as the industry was still experimenting with ideas about controllers. Nowadays controls are for the most part more settled, although there still comes up stuff like motion controls, camera integration and the like. Multiplats cannot really use them or only superficial, exclusives can lean into it. One example is Another Code: Two Memories (called Trace Memory in America) heavily uses features of the DS for it's puzzles. That makes for some unique puzzles. It also means it was impossible to port it. Which is shown by the remake in Another Code Recollection: some of the puzzles had to be heavily modified or outright replaced with new ones.
Multiplats are better for consumers: they can decide for the hardware which offers the most convenience and be sure to get these titles. If they own multiple platforms, they can decide for each game which platform serves them most. For instance if you own a Switch and a Xbox Series X, you can buy games for which you want best graphics on the Xbox and games which you want to play on the go for the Switch.
Multiplats are good for 3rd-parties. For low porting costs they can access more customers. This is not only a question of money returns, this also helps to build a fan base for the game series. This may have been different in the past. As tools weren't as advanced and computing architecture more diverse, porting costs were much higher than today. Sometimes ports were entirely new games made from the ground up. In this times exclusives may have been more lucrative, especially if the platform holder gave additional benefits (moneyhat).
Now, new technology often can be better pushed with exclusives. We see that with early VR, but by now VR games often support multiple VR headsets. For instance No Man's Sky VR supports multiple big VR platforms, which helps them get more customers in this still small market.
I am not sure that innovative ideas are pushed with exclusives like signified above. The risk-averseness of the industry comes from inflated budgets, not multiplats. So we see indies with lower budgets and lower risks to experiment more freely, but they support often as much platforms as they can.
There is though an argument for library diversification. Platform holders have an interest to broaden the appeal of their platform. This first includes common genres, but later on you want to cover more niches to get more customers. This advantage goes away with multiplat, because the intention is to broaden the appeal of the platform. So exclusives might cover more niches. Tomodachi Life, Patapon or Viva Pinata come to mind as examples for niche games that broaden the appeal of the platform.
