No, you don't have to enter them, but most of do as it's how the rescue raises their funds. We pay a pound a class (I think).
99% of us don't take it seriously, it's all for fun, and a bonus if you do get a rosette
We take the racing events more seriously, we are worse than a bunch of screaming parents on their kid's sports day
It's really laid back and easy going, so no worries if you enter or not.