If you typed those lines, then issued a

    LIST

command, the interpreter would return

    10 PRINT "1"
    20 GOTO 10
    30 PRINT "2"

You don't have to type LIST to reorder the lines, that just shows the re-ordering to you. If you didn't type LIST, but did type RUN (with the "unordered" listing) the interpreter would run each line in sequence of line number.

If you ran out of line numbers you could sometimes RENUMber the listing.

Here's Stackoverflow:

This question is marked as a duplicate: http://stackoverflow.com/questions/2435488/why-basic-had-num...

Here's the closed question it's marked as a duplicate of: http://stackoverflow.com/questions/541421/why-did-we-bother-...