Actual webserver may not be useful but it would open the door to out of the box support for servicing websockets which currently requires using things like Swoole to achieve in PHP.
Websockets are nice, but don't really fit into PHP's architecture around short lived requests.
Sure, for most parts PHP can handle that, but for long running things some I/O event based context switching typically is more efficient and in a few error cases PHP deferrs cleanup to "request shutdown" which might be a while out with websockets.
In the end PHP is focussed on one role (serving "normal" HTTP requests in isolated contexts quickly) and other technologies do other things better. And I think focus is a good thing.
Then again: PHP is open and does whatever the contributors want it to do and I myself am not actively contributing much anymore :)
