Firstly, this is not the system cache. The system cache is a cache that resides in memory and occupies physical memory that is not otherwise used by applications.
This is a .cache folder inside your home directory, so not system-wide at all. You can, if you need the space, safely delete it (only a misbehaving program would die if you delete stuff it's put in ~/.cache).
To answer your question though, deja-dup is a backup tool provided by Ubuntu. The cache probably helps it keep track of all the files in your backup sets. It probably saves a fair bit of time re-generating this information each time the backup process starts. Every file in your backup sets, it appears, has signatures stored in a cache.
5G does sound like a lot, though. If you don't use deja-dup, or you don't care about it losing its cache and being a lot more slow next time it starts, you could delete it.