Tapatalk is a pretty simple affair but it has some nice ease-of-use advantages, some of which Ali has mentioned above. I like the way the it presents the posts - much, much better for me than a browser based version on a phone or tablet. It also lets you download high res images extremely easily. Just a better experience than a browser for me. Of course, you could probably write a better app than this in a few days if you have any Swift or Objective-C skills but it's very niche so not surprising people don't.
I only use Tapatalk for this site and classiczcars.com because, as Jon says, not many sites support it.
Most apps such as this are really not adding much apart from better styling, presentation and some data organisation usually. Some people aren't at all concerned about that as witnessed by some comments saying that they found it pointless but others do find it valuable. Variety is good. It would be a real shame if Tapatalk access was taken away from Zclub.net but I get it if only a few use the facility and it's causing issues. It will definitely mean that I access the forums less purely because of how I use apps - generalised tools such as browsers slow access down for me and make it so that I don't access sites anywhere near as much as when I have a dedicated app.