It's going to vary from person to person, but mainly I think a lot of people just use the internet through their phone and they think they need an app for it.
There are a few things though, for example Tusky and Fedilab and some other apps have post scheduling which is not available on the website. Post drafts too.
You no longer need the columns for hashtags by the way, there is hashtag following now which goes in the Home timeline (more info: https://fedi.tips/how-do-i-follow-hashtags-on-mastodon-and-the-fediverse/).