If you can answer this question for Youtube and TikTok, you have already answered it for porn tube sites as well. Other categories would probably be erotic themed chat apps, social networks and of course games. And the reasons are just as valid or invalid as for the respective non-porn equivalents.

As far as I'm concerned, almost all apps could be websites.

And what do you actually get from the YouTube app that you can’t do from the website especially now that Safari finally supports notifications from the web? The same with chat apps?

I prefer using the Facebook website over the native app because the in app webview doesn’t support ad block.

I often prefer webapps as well, for instance because I can open multiple pages to keep state I meant to come back to.

But all the same reasons why some apps need native features and why some people prefer native apps are just as valid for porn. It’s not my personal preference.