top sororities

No it doesn't, it absolutely shows that a sorority is well organized to some extent and is likely top because because of that, but in itself does not define a top sorority or a bottom one. Top tier sorority is more based on are the girls friendly to each other, do they party, can they be classy when required, are they well organized, etc.

It's important for the frats for the sororities to put on a good show to sell more tickets and keep the event relevant. Like the comment above it shows that the sorority is enthusiastic about more than just partying and have a well organized exec/involved girls.

Most of the audience who goes to these events is in the Greek Scene. It's one of the few opportunities frats & sororitys have to show off what they're like aside from all of the gossip about individual members and stereotypes. So trying to win is good for your image but also it's for charity so why not put in a little effort? It sucks when no one cares or shows up to your philanthropy events so it makes the hosts have more respect for you.

I don't think winning is the only reason a sorority should be considered "top" but it's a major factor because of these reasons ^^. There's not a whole lotta chances to prove yourself so placing should count.