Why does CCleaner need to run in the background and can I stop this?

CCleaner's Smart Cleaning feature runs as a background process (if it's enabled): 

CCleaner’s Smart Cleaning feature checks the junk levels on your PC and alerts you when there is a certain amount you can clean in gigabytes. 

Smart Cleaning gives you cleaning alerts when CCleaner isn't open:

This feature runs as a background process to ensure you receive these alerts without the need to keep the CCleaner app open on your PC screen.

How to turn off the Smart Cleaning feature:

Switch off Smart Cleaning by unticking the checkboxes in the Smart Cleaning menu. 

To do this, go to Options > Smart Cleaning > Settings for Smart Cleaning.

If you disable Smart Cleaning, the background process closes and doesn't run the next time you start Windows.

Find out more about Smart Cleaning.

These instructions relate to CCleaner v5.46 and onwards. If you are not on this version, please update CCleaner and follow the instructions above.