Although this will be a biased reply as I'm a moderator, even before I was a moderator I definitely felt the Tubebuddy experience was definitely worth it. I wouldn't be moderating for something I don't believe in or have a passion for.
TubeBuddy isn't just for growth, research or better SEO practices, even though it definitely helps in all those regards, but it's also a tool that helps get tasks done quicker.
Long before YouTube brought in comment filtering (they brought it in around October 2019) I was using the TubeBuddy comment filtering and sorting to easily sort through comments I'd already replied to, and filtered it so it would only show me comments I needed to reply to. The old YouTube method was incredibly limited and resulted in a lot of scrolling and just hoping eventually you could eventually find a comment you hadn't replied to in among all the replies. The TubeBuddy filter quickly hid all the replied comments, so I could focus specifically on comments that I may have missed replying to. That in itself is an invaluable tool.
If you're a mobile only creator this is especially powerful as the YouTube studio app is still extremely limited in it's comment sorting, and the TubeBuddy app allows you to sort through comments you've replied to, hide them from your own view so you can focus on replying to those you haven't replied to. Such a powerful tool.
There's of course the keyword research tools which has helped me get better ranking keywords. You can test out tag ideas to see if they will actually perform.
A/B testing for me who's into art, super valuable, and it gives you way more data to know weather a thumbnail change has actually resulted in better performance, or worse performance. Thumbnails are the gateway to your video, if your thumbnail stinks you're less likely to get a view. You can look at a thumbnail and think "hmm... I'm not sure if this is working" and try a different thumbnail instead, use the A/B feature (legend account) and get real world data on if that thumbnail change has impacted in positive or negative views. The powerful thing here isn't necessarily the growth element, but it can also show a warning... if a new thumbnail you're trying ISN'T getting the results you hoped it would (happened to me a few times) knowing that data can prevent you from dropping CTR long term. Without that data, you THINK your thumbnail is performing better because it's "new" but in actual fact it's performing worse, and unless you manually check your CTR after a while, you don't know. If you never check your CTR and just assume a new thumbnail will work, you could be living with a lower CTR thumbnail which will be costing you potential views. A/B test results are super important in that regard.
You also have to realise that not all topics are worth doing a video on. you can get a 100/100 tag score, but if the topic isn't interesting to people, people won't be interested even if your SEO is great, this is why it's good to do keyword testing and exploring BEFORE making a video.
