preventer, yes!
Keeps the boom from flying from one side to the other, during accidental Jibing.

Before I knew what mine did, i was told it was a Vang, mostly racing stuff. So I took it off. Sailing in light to medium wind no problem. But in big wind, handy to have! One less thing to worry as much about while dealing with strong unstable winds.

Many boats to not have one, so it is your call. I ran with mine off for a year, after I needed it I put back on and kept it. Came in handly, but I'm still a toddler sailor