What can you do with REALLY old computers?

My retro computing passion doesn't really kick in until the 1980s because that seems to be when home computers exploded in popularity. There are a lot of games for these systems, of course, but also a lot of office and graphics applications. I've recently been watching videos about older, or at least non-home, computers, and the appeal is harder for me to understand because, honestly, I don't know what you can do with these machines. Why might someone get into repairing and running them? What can a business computer from the 1970s do besides basic data entry and retrieval? If you set up a terminal and a... whatever the actual "brain" of the computer that the terminal connects to is called, what kinds of programs can it run? Or is the appeal less about what these machines *do* and more about the electronics and hardware inside? I'm not denying the historical importance of keeping this old hardware running; I'm just looking at it as someone who has at least a little knowledge of what home computers of the 80s and 90s can do, but little to no knowledge about what 70s home computers and business computers can do. They must have had important functions to justify how expensive they were.

No judgement here! Genuinely curious! :)