The idea is that there are different things that different languages offer, and you need to know "what's out there" before you can claim that one language is the best.

For example, if you never programmed in any of statically-typed languages, you cannot say if they are better or worse. Similar things go for OOP-oriented stuff, or immutable-by-default languages or languages with well-designed stdlib, etc...

You may still think that PHP is the best language after trying a few other, wildly different ones, and that's fine. But unfortunately, in my experience, a lot of people who really advertise PHP (and some other languages like Pascal), haven't really tried the alternatives.