Instead of giving you another downvote, I'll give you a reply. I think you're asking a legitimate question and your statement is a valid stance. Up front: I use boost and have paid to remove the ads.

Why would people want to use Lemmy, but in a way that has ads?

I have a hard time wrapping my head around that, too. But, I have met people who don't mind ads who loosely fall into three categories:

1. A lot of people grew up with everything being ad-infested and are simply not aware of options that are ad-free or to remove ads. They can't grasp the idea to pay money to remove ads, because they don't value an ad-free experience enough to justify such an investment. After all, ads are everywhere and "always have been". Even if you tell them about things like adblock, they'll usually not look into it and forget about it, because they do not care! Such people are usually not aware of things like ads as an attack vector.

2. Some people have an internal filter: Their minds are so trained to finding the actual content on the web, in news articles, apps, ... that they subconciously just skip all the ads. I once asked a colleague how he can tolerate so many ads, while skimming over an article. His reply: "There were ads?". I met such people once or twice more.

3. Some people are aware of risks and technical problems of ads, and know about things like adblock, but deliberstely choose not to disable ads out of conviction. The thought process isn't even that hard to grasp: People need money, I just consumed something that somebody invested time into and they get the money from ad providers so I don't have to pay. Suppressing the ads is effectively theft.

I don't get why people want an app for every website, just use the website

Lot's of websites are really clunky on mobile. Most websites transfer north of ten times the data of the content you actually want to see (ads, JS frameworks, header, footer, ...). Apps often offer a smoother experience for the same content on mobile. Also, people have been trained to expect/want an app for every site. I find it especially egregious when I see developers having multiple proprietary "authenticaor apps" (Microsoft, Sophos, ...), and I'm like: You know those are all just TOTP clients and you could just use one open source app for all of that (FreeOTP for example). Those do not replace websites, though.

My personal stance (everything before this was just observation): First of all: I do not see ads. I either pay to not see them or block them. I use as much open source software as I can. But, I acknowledge that sometimes there are proprietary solutions which are simply better then the open source counterparts, or, open source solutions which generate income via paid for "premium features". I'm willing to pay if the other side seems to have tolerable business practices and I think my benefit is worth it. Software I pay/paid for: JetBrains all products pack, Bitwarden, Simple * apps for android (for example Simple Gallery), Boost for reddit, Boost for lemmy. The list is incomplete, but you should get the idea.