Paola, your project is being saved on your computer. If you upload via the WX5 built in FTP client then a backup will be made by default (click on down arrow next to Save to verify), and if you export the project you have the perfect backup.

No it should not be necessary to pay for a one.com backup if you simply wish to restore your website. However, in the event of a catastrophy you will again have to setup your email account/s, you will loose any long term stats and if you store anything else in your hosting space (eg databases, misc files, etc) they will also be lost.

I do not pay for my backups.  If I loose everything online I know I can restore my websites and reconfigure mail, etc. without too much grief.