If you're already writing all callouts from your controllers in a blocking manner then nothing. Because async IO is a pain in the rear lots of libraries try to

If you're already writing all callouts from your controllers in a blocking manner then nothing. Because async IO is a pain in the rear lots of libraries try to hide it from you in various ways. However if you're using e.g. pure Java JDBC / SQL drivers, then you should be able to get higher throughput from your web servers and can maybe shrink your server footprint. However, lots of people don't write code that is just HTTP servers ("microservices"). Also, consider that when it gets easier to write server frameworks there'll be more competition and they'll move faster. So maybe Spring Boot won't seem so attractive in future as someone will have developed something better.