The number one reason home computers fail, is they are turned off when not in use. Computers generate heat, internal temperature of the CPU can exceed 158F degrees , possibly higher. Hard disks platters that are inside the hard drive spin between 5200 rpm and 7200 rpm, on consumer products. Fans rotate, at a low speed but they collect dust and grease in the air that sticks to their blades. At the point you shut them down, they spin down to their off state, the dust and grease collect on the fans, eventually acting like glue, adding extra resistance, so they work harder those fans die, there is rarely any alerts that the fans have failed making the system hotter. Metal and Silicon that make up your system don’t really like heat up and then cool down cycles causing them to die sooner. The hardest part of a hard motor life is when it has to start from 0 rpm and get to 5200 or even 7200 rpm quickly.
I keep most of my systems on 24/7 365. I haven’t had a CPU, motherboard, or memory fail in years, though i do usually buy ECC ram that is designed to last longer and protect its contents better. Hard drives usually last years for me, I have had like 1 drive fail in the last 6 years, and I do have more than 20 harddrives in use on my personal systems. For most systems I am ready to retire them long before I see a failure in them.
Now is it cheaper to keep your computer running at idle 24/7, or pay for repairs that depends on your usage pattern, how long you keep your computers, and the cost of your power.
In my work life, I deal with many 1000’s of systems that are kept on 24/7 for years at a time, eventually because of manufacturer defects or vibrations they are in racks with dozens of other systems, Fans, power supplies, and disk drives do fail, thankfully monitoring usually catches these and as you increase your working set, you will see things that a person that only has a few will never see.