In the US, the cities with the largest parades are in order: New York (Manhattan), Chicago, Boston, Philadelphia, Cleveland. Cleveland has found a record that a St. Patrick's parade (probably a procession) took place on the west side in 1842. Seemingly all Catholic and Orthodox nations have their days of celebration. Parishes, and then groups of parishes, and then with sodalities, and then social organisations were added, then labor groups (possible development of accumulation). Well, somewhere it expanded beyond Catholics and Irish. First of all, Patrick is a saint for the entire Church. So at some point, people saw this was a vehicle to join, and to be seen.
As i wrote two posts ago: always there are old cars, horses, dogs, bagpipers, high school bands. People expect and enjoy seeing them. Then there are novelty groups that are well received. The Ghostbusters, and the Star Wars costumers were there again. The comic book heroes were not there this year.
Back to the Future appeared this year. Doctor Emmet Brown retrofitted a DMC DeLorean into a time machine.
All cars go for bomb testing before being allowed in the parade. There is a sniffer dog, and a pair of mine sweepers (i guess). Some of these cars have in the parade for years. Are you going to blow up your 1950 Ford Custom, or some other car you keep immaculate, that some people have fits if it is smudged or scratched, even imaginary defects?
When then Vice-President Joseph Biden was in Ukraine in 2009, he saw a squad of female soldiers march. He remarked how beautiful Ukrainian women were, it caused a minor kerfuffle at the time.