Back in the day you didn't have a 2 dimensional editor like emacs or vi. All you had was the command line.
Your program was stored in memory and you would type in single line commands to edit single lines.
If you were a Unix god you could do it with ed or something, but for BASIC on a C-64, VIC-20, or TRS-80 you'd just overwrite the line.
So a session might look like:
$10 PRINT "Hellow World"
$20 GOTO 10
$10 PRINT "Hello World"
And now the program would work correctly.
Some older mainframes even had line terminals without a screen. Your whole session was printed on paper in ink!
