I don't know if it's good for the first comment, because my answer is from personal experience. But i'm not sure there are historical sources for this question, so I'll try.

I can tell you about 1980s. It is about parades and demonstrations because mechanics and motivations were the same.

I believe there were two main reasons. First, a lot of people were forced to participate by their employers. Youngsters were forced to go by schools. Actually it was relatively easy to skip but in some cases you had to explain why you didn't attend — overslept, was sick and so on. Also there was a perception that it's not very good for you career to skip such events on a regular basis.

The second reason — it was some sort of entertainment. I don't think that attending the parade is fun but I know a lot of people who like it, and even now these people are watching TV translations of the parades. They just like it. There are military cars. And missiles. And so on. Also you can chat with your friends and colleagues all the time.

The closest analogue for this, I believe, is a team-building event. It is expected that you will attend, and often it's boring as hell, but some people just like it for some reasons.

There were only 4 to 5 major parades a year, usually on a holiday. Also they were entertaining. Most people probably enjoyed them