The explicitly positive integer concept is something taken from SQL. Here is a link from the Oracle SQL database docs since it includes a nice infographic, but you can also check out the SQL paper (1992) and search for "<signed integer literal>".
The main reason this exists is just for completeness, in your normal use of GQL it is ok to just leave the operator off to specify a positive number.
My best guess as to when you would actually use this is if you were building a GQL string and found it useful to add a modifier regardless of if a number is positive or negative.