If you're a guy here asking why things went wrong (or right) then you need one. As a courtesy to those answering the question, if nothing else.
Other than that, I don't know that I'd say it's essential for everyone. If you're just out to have fun making some sort of beer-like beverage, then you 'll accomplish that most of the time without hydrometers, thermometers, etc. If you want to accomplish a certain style or ensure your product comes out like you thought it might, those are the impliments that you need.
I don't golf, but I'll use that analogy.
If you're perfectly happy smacking the ball around, don't score or don't even use the correct clubs for the shot and you still have a blast, you can say that you went golfing.
The minute you start asking why your shot slices or why your ball rolled father than you wanted because you used a 3 instead of a 9, well, then I think it's time to employ the tools (knowledge) available.