Do you sing, play an instrument, or write poetry ? Are you a band ? Why should you consider participating in an open mic ?
Meet other artists and maybe build a new project with them
Do you play alone ? Why not try an open mic, even if you are "just" singing a cappella, or if you are playing the guitar without singing... You could maybe meet one or more partners and start a great new project with them !
Get experience in live events with an audience
Playing in front of people, and mainly people you don't know, is a start. The stress is there, even in front of 10 people. You will learn to manage it for 2 or 3 songs. After this, it will be easier to play in front of more people and for longer. Also, the "rapport" with the audience is easier to build with a small number of people... and is often warmer than in a giant concert. Still, we wish you the best luck to reach that stage once !
Test new songs
You could get feedback from your family and friends, of course, but they are always less objective than strangers. Careful, we advise you to always protect your work before playing in public.
Open mics are great for feedback, because the artists are close to the audience. Other artists and the audience will comment and make suggestions. Then, it's up to you to improve your songs !
If you never, or almost never, play in front of an audience, playing in an open mic will change the way people look at you (that is, if you play well !) and you will be super cool.
If you are already famous, think how super cool you would be tu turn up to an open mic, out of the blue !