Fans obviously are there to keep things cool.
Most, in any bog-standard PC run all the time.
Are you saying that yours only run occasionally?
There are more sophisticated systems and your Pavillion my be one of these, where a heat sensor governs the speed of a fan and may even switch it off if the temerature is suffiently low.
The main power supply unit fan should run all the time and probably the graphics card as well, but extra case fans might be "part time."
If you have only just noticed them "kicking in" then it might be that the innards and particularly the fans themselves need a thorough clean, also the weather is hotter now by far that it has been since last year.