How about a bunch of very easy to imagine examples:
1. You're mowing a field of 2' tall grass. You accidentally mow the owner's cat. This was not just any cat, but a $2500 fancy show cat. And the owner was very attached, now he adds emotional damages. He sues you for $10,000. What do you tell the judge? "Oopsies, I'm sorry"?
2. You're mowing that same field and throw a rock through the windshield of a vintage automobile. Can't buy a new one, all the junk yard stuff cost $2000 or more. You get to pay for it, lawsuit or not.
3. Somebody hires you to remove a tree on "his" land. Half way through the job, the neighbor comes running out and tells you that it is actually his land and you have now caused extensive damage. Who pays for this?
4. You mow somebody's yard and nothing happens. 4 days later you get a call saying you ran over something expensive and they demand you fix it. How do you prove it wasn't you? People intentionally hire people like this with the objective of getting free work or free money, it's a common scam.
5. You're mowing grass and a spark from your tractor starts a fire. Fire burns down house. Lawsuit ensues.
Remember, even if you are doing work for your best friend, and he would never, ever sue you, his homeowners insurance probably will if he has to file a claim.
Chances are, if you're doing tractor work on the side for money, then you don't have a lot of it in the bank. Do you have $2500 or more to hand over for your screw ups? Insurance would have been a lot cheaper.