It protects you from pregnancy and STD's like Syphilis which can be fatal and cause infertility, and Herpes which can never be cured and costs $300 a month to treat... Condoms are a form of protection. Although not 100% effective from preventing pregnancy, it is always safe to wear one. It also protects you from STD's (sexually transmitted diseases) so my advice to you is always wear a condom while having sexual intercourse.
In more basic terms they perform a safety barrier. They are intended to keep what is on each side of the condom seperated from the other side. People wear them to keep themselves safer. The other awnsers may explain the negatve outcomes you want to not happen.