If the PC has been in extended continuous use memory leaks can occur from some programs (usually badly written ones) that mean used memory is not released for reuse. Over time this can then accumulate.
Provided that after each reboot your memory usage drops to the same level, I wouldn't worry. Rebooting more frequently than you currently say you're doing would however probably be a good idea.
The only other possibility than the above reason that I can think of would be malware. Make sure your security software (including real-time protection) is present and up to date. Do a full scan of all drives for malware.
All answers and suggestions are provided by an enthusiastic amateur and are therefore without warranty either explicit or implicit. Basically you use my suggestions at your own risk.