Supplements a curse, a blessing or just a real pain in the ...?
A long time ago (think around 10 years ago) a college supplement was a way to collect additional data on an applicant. The data sought, looked at legacy information (did mom or dad or their parents attend the school for instance), some particular information on your choice of major or other demographic data that is not normally asked in the body of the main application. For many schools this remains the case to day.
For the purpose of this answer let's look at the Common application which represents over 400 of the top colleges and universities in the US. In addition to the above each school may want to know why you want to go to their institution and will allow you to write an essay on this subject. They want to know if you've done your homework and can cite things that are specific and unique to their school or campus. If you reply with generic terms, your response can hurt your chance for admission.
More selective schools will ask more challenging questions. Here, the essays are tougher and the supplement IS used as a key criterion for making an admission decision. You need to be careful as you reply to these questions and must give mature well thought out answers. One popular school asks you to describe something that outrages you. Trust me: telling the school about the annoying chem lab partner that loudly chews gum while you do your lab work is a sure to get tossed into the deny file. Now that many schools are test optional, a supplement becomes a necessity in order to differentiate applicants when they are in the admission decision process.
So what are they for you? As a blessing they can be the best way for you to showcase you uniqueness and increase your chances for admission. As a curse they can become onerous and the more schools that you apply to the more time and care you'll need to put into them. Yep, they're a pain, but if you are seriously into doing your best when you apply, you must take each one as a small project whose importance may make a major difference in your life.