You can manually program them just fine without a cable. The cable just makes it easier to do a lot of programming all at once.

Example: With some HAM frequencies, you broadcast at one frequency, but listen at a slightly different one. The settings for the frequency ranges needs to be programmed, then each frequency individually programmed. It's a lot easier to type all the info into a program on the computer and send all the info at once, then it is to go through the motions of entering menus, selecting, confirming, exiting menus, etc. Think of the difference between texting back when you had to push a button three times just to get to a certain letter, compared to just typing on a keyboard.

Don't NEED a cable, but it's easier.