You can redownload apps from the App Store, but app data must be backed up or it will be lost in case of disaster.
Some data does not need to be backed up if it is synced to another server, such as your email server. Email servers are the master data for Contacts, Calendars, Notes and Reminders, so if you have one or more mail accounts of for sync in Settings/Mail/Accounts, and it will list the accounts that sync these items. Some apps also keep data synced to the app provider’s servers. For these you can delete the app and reinstall without losing data. These are generally the apps that require sign-in.
Why don’t you backup your iPad? Go to Settings/[your name]/iCloud and turn on Backups. It will automatically back up overnight if you leave the iPad plugged in and connected to power.