A backup is a copy of a site that is kept on another hosting server and could be restored if something goes wrong with the live site - a failed script update, an accidental deletion of some file or of the entire database, etc. Restoring the site the way it was shall remove or limit the damage the issue may have caused, that's by all means an obviously better option than having to reconstruct the entire site from scratch. Although you can download a copy of your content on your PC, keeping a backup is a function that the majority of hosting companies include as part of their plans. It is important to take a look at how often they do that, though, because some service providers create a backup just once every week, that could be far from enough for a booking website or an e-commerce portal where the data is updated each day. Make sure that you see how fast and easy a backup can be restored, which could be essential if some problem appears on your site.