We are with PetPlan. We've had dogs for over 25 years.
It's all about cash flow. We did at one point stop insurance, and then two of our dogs got cancer, an injury requiring surgery, and a condition requiring surgery. We paid out about ₤5k in a year over 10 years ago and only one of those 2 dogs survived in the end.
We went back to insuring them, so, when a later dog got cancer aged 5 and had scans, surgery, and treatment for post surgical complications which he did not survive, our grief at losing him wasn't accompanied by the grief of losing another £5k in one lump!
Our current dog is over 10 and only 80% of vet fees are repaid under the plan but he IS still covered, which is good, because this year the insurance payouts have been roughly what we've paid in over the years, and could easily have been more. We avoided hitting the annual payout limit because his vet visits straddled insurance years.
We decided to go for the security of knowing big bills would (mostly) be covered, and having the pain of seeing the money go spread out, rather than losing a dog AND many thousands of pounds at the same time.
I know people do sometimes take the view "I wouldn't pay out more than X, I would have them PTS", but I would find that very difficult. The vast majority of the time, vet treatment for my dogs has been effective - even when it did not cure, it yielded many months if quality life - and they bounced back very quickly, much faster than humans do!
So I pay the insurance for the privilege of saying, "do whatever is best of the dog" and being able to just worry about getting them through it.
Be aware that vet costs have gone up and vets are very busy (lockdown puppies and Brexit and COVID) so shopping around may introduce significant delays.
Dogs are expensive! Mine now costs ~£3k/year (old-dog food, old-dog insurance, jabs, excess vet fees, kennels, treats).