Non-magnetic data storage medium which allowed offline source code editing without the need to learn how to use an editor. Easy (if bulky) to store and handle.
The 80 characters on regular punched cards are the reason for today's 80 columns in text mode terminals and on most other non-graphical displays. Punch cards usually used a code that was non-binary to store textual information.
Even when the first terminals came around (i.e. IBM 3270), very few installations would want to waste the cpu cycles and memory to run something like editing on their mainframe. (A very large system in the mid-70s would have something like four megabytes of physical memory for about 100 users.)