They were essential for branching with GOTO and GOSUB.

Example:

    10 PRINT "HELLO ";
    20 GOTO 10

This would create an infinite loop that you could break with Ctrl+C.

You could then type:

    15 PRINT "WORLD ";

And when you listed the source code (with the command LIST) you would see:

    10 PRINT "HELLO ";
    15 PRINT "WORLD ";
    20 GOTO 10