If the service is paid, they can hire people for this service. No need to take resources out of the engine development team. Also, does it have to be Epic? Could be another company providing the service.
Well, for a small team it might be interesting. Same reason IAAS/PAAS/SAAS/etc exists these days. Nothing easier than dumping the stuff at Azure/Amazon/Google and let them handle everything. I dont want to maintain my webserver: handling log files, keeping OS up to date, keeping web server software up to date, worrying about hardware replacement when its too old or is broken etc.
If I dont need to maintain a build server, I have time to work on the game. In the end it could be cheaper than having my own hardware for building. And a build server in the cloud opens up the possibility for scaling etc. If the game grows it will have longer build times. If you need speed, you need to buy more hardware, install it, maintain it etc. In the cloud I can say ‘run the build on 6 servers’ and be done with it.
So from my view: If your project becomes big/complex, you might actually want an online build service!