What's the point of it? Is it to raise awareness of the vulnerability of cyclists or to raise money for charity etc? I wouldn't do it but each to their own.
A mix of things. Not raising money.
Three or so main aims.
Raise awareness of cyclist vulnerability is one of them.
Protest about over use of/ dependence on oil products.
Promoting positive body image.
Initially, the second of those was the main point, but over the years, the first has become the most important.
And yes, as
@numbnuts says, a couple of hours of fun and laughter. Usually with an almost entirely positive reaction from bystanders.