how do i remove appdata ?

For a particular app only: Settings > Storage & USB > Internal Storage > Apps > (select the app).

For ALL apps: Settings > Storage & USB > Internal Storage > Cached Data.
 
Ptkelly, because you're giving the phone to someone else - or because you messed up the data and the app doesn't let you change it, usually.
 
A couple of apps didn't work when my phone updated to Nougat. Clearing those app's data solved the problem.