I test because it gives me a sense of accomplishment when I pass a test and also something to work towards - a tangible goal that gives me something to focus on in lessons and in my practice sessions.
I feel like I have accomplished more while I've been actively working towards tests than I did while I wasn't. I think about it this way - just one year ago I couldn't complete all of the moves on silver (the stamina/endurance for the spirals was only 'almost' there and the 3's in the field were sometimes there, sometimes not even close, and I would often just work on the turns on a hockey line, not even in pattern because the pattern stressed me out), yet I was able to successfully take and pass that test with only 6 months of serious "work". If I hadn't set testing them as a goal, I'd probably still not be able to do those moves now. It's been the same with gold moves. After I passed silver I went from sometimes being able to do the brackets in the field without bailing and two footing the turns, or at the very least touching a toe down on the back turns, to being able to complete the pattern with all clean turns almost every time I do it in just a couple of months. A few months later (now) and I'm almost ready to test gold - it's actually one of the double 3's that trips me up, but I can do it fine, it's more of a mental thing (aka fear of falling) that is stopping me at this point, and I just need to get over that and go for it.
My thoughts are, there is NO REASON that you can't work towards a test, even if you don't ever actually test, but you can also view testing (and passing a test) as the "reward" you get for all of your hard work. As someone who has zero interest in competing, it's all the validation my skating will get
My coach and I have actually had discussions about "what's next" if/when I pass my adult golds (I've already told her we'll focus on my bronze and silver freeskate tests once I pass gold moves), and if/when that happens I'll probably continue working on MITF, but I can't see myself pushing for any freeskate levels beyond gold - if I am even able to get an axel or double back to pass that one.