Full disclosure, I'm a YouTuber with a Patreon account, so I've got a horse in this race. But one thing to add in here in a bit more detail is that YouTube revenue (from ads or the folks who sign up for YouTube Red) is relatively small, and relatively delayed, relative to what a lot of folks expect.
YouTube doesn't want you to be too precise about numbers, I gather, but even with around 15,000 subs, I have never had a video which "paid for" the purchase of an item outright, and only rarely does it cover the loss incurred when buying and then later selling an item. Thanks to generous viewers, I get a lot of loaned gear for review (which is the only thing that's made my channel possible), and a heavy majority (70-80%) of my review videos haven't actually made enough money since release to cover insured return shipping of the item, and right now, the median per-video YouTube revenue won't even cover a flat-rate envelope.
That will (hopefully) change over time, as the videos have more time to get views. But right now, when put next to the time spent on the channel, I'm not quite making minimum wage from YouTube, and that completely ignores money spent on buying gear outright, shipping, etc.
Along with gear loans which make my channel possible at all, Patreon is the glue that helps this make any sense. Shipping, purchase, etc costs are all front-loaded, and YouTube Revenue rolls in over months. The patreon money, donated regularly and with minimal added costs, allows me to pay shipping, allows me to buy a bit more gear, and allows me to pick up a much-anticipated piece of gear, knowing full well that I'm going to take a hit if I re-sell, and provide that content. I have a day job, so i don't need to make a living from YouTube, but for me, the support I get via Patreon is a buffer between the needs of the channel and my personal bank account, and it helps the channel feel like it's not eating my financial face.
If you don't have the means to support a channel you love on Patreon, I totally get it, there's zero pressure. Just enjoy the content. That's why we're doing it: for you to enjoy. The ad revenue from your view will help a bit, but a kind comment or message or email is great payment too. But if you have the means to help support the channels you enjoy with Patreon, I encourage you to. It really is a big help!