Formatting a drive dries all data out of it, giving you a clean slate to start over. It also allows you to change the file system on the drive. You may need to change this to use the drive with another device or if an app does not support the default file system. If you've been affected by OneDrive's last rejection to sync with non-NTFS drives, follow this guide to reformat an SD card to use NTFS.
Formatting a drive is a simple procedure. As a final reminder, before you get started, formatting permanently deletes all you have saved on the drive, permanently . You can not restore your files later unless you have made a backup first.