Why would the thermostat be removed

If someone was having a problem with overheating and did it as a test to see if it would stop overheating without it.
If there was no new thermostat to install at the time and the old one was bad.
You should have one installed so the pistons and rings expand when they should, so the engine warms up as it should and so you have heat when needed. Not to mention without one you will get a lot of blowby past the piston rings which can dilute the motor oil with gasoline.