It's possible that the BookedIN app did not have enough device storage space to save your sign in information at one point.
First, check to see how much space is available on your iPhone or Android device. Try freeing up some space by clearing out unwanted videos, apps, photos, or just follow these handy iPhone or Android tips.
Once you've freed up some space, fully close/quit the BookedIN app.
Now reopen the BookedIN app and login. It should now save your sign in information and you won't have to sign in every time.