The concept of a "Software Interrupt" (or "Trap") is a very common way of calling operating-system type functions; eg, MS-DOS & the PC BIOS rely upon it!

The advantage is that the "caller" does not need to know the address of the function, so it does not need to be linked with the OS or a library - it just issues "INT 21" or whatever. This is also very compact, as the full called address doesn't have to be specified, just the interrupt number.

But you are right: on the 8051, one can simply set the appropriate hardware flag to invoke an interrupt from software.