Who would pay for someone to host their blockland server in the first place. Costs less to just leave your computer on.
You obviously don't understand how computers and servers work.
People do this so they don't have to keep their PCs on all day which in reality uses more electricity and rises your Electricity bill, and it then costs more then a Dedi hosting service.
They also do It so their server doesn't lag if they have a bad PC or Internet Supplier or use the PC for other things.