Unlike Node, Python, Java, etc. that start a long-lived server, each request is handled by PHP in a separate process.
That's what makes it really to use with serverless (and lift-and-shift, like what Treezor did).
Comparing Bref (https://bref.sh/docs#maturity-matrix) which inherits Lambda's 200ms+ cold starts vs Cloudflare's 5ms (https://blog.cloudflare.com/eliminating-cold-starts-with-clo...), for example, is a pretty big difference.
But I guess if performance isn't a critical use (or you just have so many invocations all the time in every data center that cold start isn't an issue), it's nice to be able to ~port~ copy and paste over your existing PHP code.