I see people posting big home server racks and have always been curious
Pure fun and experimentation. It can help build/refine skills that are in-demand, but I doubt many/any homelabs actually make money. In reality, most probably cost quite a bit.
It’s just a hobby like restoring/modifying a car, building stuff in a shop, sewing/knitting, etc. People make money with those skills, but some people just do it for fun.
Very highly performing systems can simulate many much more regular powered systems. Generally these people are condensing the function of many low performing computers into just one. He can have a media server, home security camera server, torrenting server, minecraft server, data backup server, nas server etc, smart home automation server. All these virtual machines and more provide an incredible learning opportunity for any home lab beginner. Cheap servers on eBay can be a steady source of processing power when consumer products are in such short supply
You have to keep in mind that technology like servers can lose value extremely quickly.
Businesses upgrade their equipment regularly. You might think that once you bought a server it would essentially free to keep it running, but electricity and cooling and them simply taking up space mean that after a few years it can easily be cheaper to turn them off and buy new ones than to keep them around. (Especially if the business is smart enough to only use hardware that is within its warranty for anything the business needs to keep making money.)
Also there has been a lot of consolidation, virtualization and moving things to the cloud going on, meaning that many companies now need less on premises hardware than they needed years ago.
All that adds up to lots of used server hardware being around for people to use at home and try things out.
some of the low end server hardware can be as cheap as high end desktop computer.
Some people simply have enough money to spend on a hobby of theirs to buy new servers.
They will almost never be able to make a profit of them though. Renting servers in the cloud from amazon or Microsoft is so cheap that your homelab will almost never be a cheaper alternative.
It does give you an opportunity to try all sorts of things though.
A lot of people (myself included), don’t have home racks to make money. That never even enters the equation.
I have a home rack for training (I work in IT), for mass storage (films and TV shows, software – keeping it internal to my home network means I don’t rely on the internet being online), and for failover networking (working from home and home “smart” devices mean constant uptime is a necessity).
And as somebody else said, old hardware is cheap (comparatively).
My brother is in the process of building a home rack. He is a software developer and being able to have various environments to test and learn on are helpful for his profession. He purchases older equipment that still performs well and the price point is much lower than brand new gear.