It is expected that the amount of CPU time is not too long in order not to degrade the performance of other programs that might be running in the same computer.
When all programs are waiting for user response or for some event to occur, the operating system executes the so called idle cycle which does not perform anything. It normally executes some processor instruction that makes the CPU run cooler until some event happens.
The programs for distributed computing projects run all the time thus there is no idle cycle. In order not to degrade the operation of other programs, they run at a lower priority, so these programs can respond quickly to user requirements.