I don't know other languages/VMs that requires to set JVM's Xmx/Xms equivalent parameter. Why only JVM requires it? What about to just set unlimited by default?

Generally, you want to leave some memory available to other things, like the OS, buffers in the network stack, etc.

Having a limit for the VM is helpful. Also, by default it is automatically 20% of available memory.

Personally, I've had to tune this kind of stuff with every VM I worked with (JVM, Node, PHP). :)

I'm not a Java dev at all, and I've tweaked more than just those parameters. Simply running Java programs is sufficient that you can end up introduced to the JVM's GC.