Stove thermometers are designed to offer a guide as to the temperature that your stove is burning at. If your stove is not hot enough, then you risk increased creosote and tar build up. If your stove is burning too hot, then you are using more fuel than you need to be.
Are They Accurate?
Stove thermometers are certainly not precision instruments by any means. They do offer a handy guide however. How else do you know how hot your stove is burning without one?
We would say that it's one of those things that, once you have one, you wonder how you managed without it.