The assembler ("compiler") needs to distinguish symbols (labels, names), such as LOOP1 from literals (numbers), such as 123h.

The examples above are pretty obvious, but what if I have a label
AAh:
and also want to use the number, AAh?

The agreed way how to distinguish symbols from numbers in Intel's notation (i.e. with the 'h' suffix to denote hexadecimal number) is, that:
- labels start with characters ('a'-'z'; 'A'-'Z') and underscore ('_')
- numbers start with digits ('0'-'9').

That's why a '0' has to be written explicitly in front of such hexacedimal numbers, which start with "letters" (A-F).

JW


PS. It's compiler, not complier.