Backing up your website is important. Daily backups for a website with a blog that is updated frequently is even more important. Now when you’re doing backups that often, you would want to make sure that you’re not backing up the wrong data or unnecessary data.
Why? Well every backup you take costs you. It costs you in terms of time and money. If your backups aren’t optimal, the time it takes to backup your site increases considerably as well as the space your backups occupy, either on your server or on the cloud ( Dropbox, Amazon S3 ). Now as you are well aware, that spaces costs money.

So, which files should you not be backing up?

Cache files

Cache files are static html pages that are snapshots of your dynamic website. They’re just temporary copies of your pages. These files are served to visitors instead of the same dynamic page being generated for each visitor. Cache files significantly reduce the load on your WordPress database.
But cache files don’t need to be backed up. They eat up valuable storage space. Which is why you must look out for them and exclude them from your backups.

Backup files by other plugins

There are a lot of free backup plugins around. Installing them and then forgetting about them is quite common. A lot of these plugins save files on your web server. These files shouldn’t be backed up, so look out for them.

Error log files

Error log files can be pretty large and can consume a lot of space. Log files being dynamic, ie.. their continuously updated, there is no need to backup an old error log file. Search for these and don’t backup these files.

Now here’s the problem, what do these files look like? How am I sure that I’m excluding the right file from my backup? It depends alot on the plugin you’ve used for backups and caching because they store data under their own folders. This takes practice and experience. If you couldn’t be bothered with it, use a WordPress Backup plugin that does it for you and make sure that they value your time and money by excluding these files from your backups.