I was taken to an allergist when I was about 3; one of my earliest memories was crying at his office (because of course back then, testing was one tiny injection right after another). I saw him again when I was about 10 and his bedside manner sucked mightily (like, walking up to me, shoving a cotton swab up my nose, leaving it there and walking away, without a word to me). All in all, this lead to years of nonstop allergy shots - which may or may not have done much to help me.
As an adult, I voluntarily went through similar testing because I was hoping that they could do a round of shots to reduce some of my environmental allergies. I think that ultimately did help, though it’s a very slow process.
To my mind, the best use of the testing is to help identify allergies you aren’t sure you have. I mean, if you start sneezin’ and wheezin’ when you’re around a cat for 10 minutes, you don’t need to be tested. But if you’re having major symptoms around the house, is it the flowers in bloom outside? mold in the air conditioner? the dog? Then you have the option of eliminating the source, if possible, or seeking treatment to reduce sensitivity.
As I understand it, food allergy testing is still hit or miss. My son had a very definite reaction to eating peanuts as a a toddler - definite enough (per my report) that the doctor wouldn’t even conduct skin testing for peanuts. She did test for other foods and identified a number of possible culprits (soy, chicken etc.) that he’d never had an issue with. We did an elimination diet followed by slowly reintroducing those, and he was fine.
Blood testing was either very new, or nonexistent (this was 23 or so years ago). Next go-round, when he was about 12, they did blood testing which suggested some of things from the previous round were not problems.
I haven’t been to an allergist in 23 or so years - last time I saw mine was when I was pregnant with my daughter. I haven’t felt like investing in a course of shots, and I haven’t had any inexplicable symptoms, so I haven’t felt the need. If they had a better track record for dealing with cat allergies, I’d be all for it!!
Oh, and I do think the series I had as an adult (in my mid-20s and again in my mid 30s) did help with the “hay fever”-type of allergies; those are substantially better than they were before I started them. I don’t have enough faith in the cat component to risk getting one, though.