So I use the USMLE decks even though i'm not sitting the steps. I find the biggest benefit from having the anki decks is that it's basically all the important information at your finger tips, I don't have to sift through 100+ lecture slides when I know they're going to ask 2 questions from that lecture. For some context I have 6 weeks of lectures total this year and we're given a list of 400 diseases/conditions etc to know as if we were an F1 (1st year resident). It's nearly impossible to meet that expectation with only 6 weeks of lectures so you have to use external resources and people have already done the hardwork of taking whats relevant and made it into the anki decks.
You don't have to use anki in the way everyone here suggests, personally I think it's incredibly nice to have and use as basically a dictionary/bible for example if i'm on placement I can quickly open anki and search for the relevant tags to whatever specialty i'm on and quickly scroll through the cards. Usually i'll ask the attending if there's anything they would like me to focus on over the next few days (aka what cases do you have) and i'll simply Anki those during breakfast and be able to adequately answer any pimping.
I'm the only person in my class AFAIK who uses cards (people know this and ask me what are your cards for say PMR) and I do quite well but the key difference is people seem to spend a lot longer than I do studying for maybe an extra 2-5% which is usually luck based on the 50-50 questions anyway or material we would've had to luckily come across on placement. So I find it just to be overall more effective for basically the reason I put in the first paragraph.
Of course you can score amazing without using Anki several people from my school have got 245+ step 1's without using it. If you ask me the key to any exam no matter the subject is practice questions. Those who do more questions generally score higher, I have no data for this but to me it just seems common sense, which is why people tell you that you have to do Uworld etc and you can't just do the anki decks alone.
P.s I do use the anki decks as intended, I do my cards everyday but seriously it's so useful to type in whatever your attending says you've got that day than to read through 100+ slides. I didn't always use cards either but since I started i've had a much more enjoyable time in med school due to the excess free time it's created.