A lot of people use reddit/Lemmy on the transit to work/school/university, and on mobile the official reddit app stinks, especially the video player. Other then that third party apps provide a lot of customisation, for example filters and different displays methods. What you see on desktop is a card view by default and list view when using the old website or compact view, on for example “boost for reddit” you could have cards, list, gallery and swap views. You could have a different view and filters set for each sub, set it so when you save the post it automatically up votes it and much more.
Basically it was mostly because people like to have something personal, customised perfectly to thier needs and wants while the official reddit apps just didn’t provide enough.