You don't need docker to run a single program on a single computer... but the moment you either want to run multiple copies, or distribute it to multiple computers, particularly if they're not your computers, then being able to encapsulate stuff and reduce the number of dependencies is a huge benefit.

For example, it lets you run software on your computer without worrying about installing stuff that might affect other software, and without even having any interactions with other software aside from what you allow it.