Free - as in "doesn't exist"
My major reason for using free / libre / gratis software is quite simple.
Most software is crap. I can pay for software - and it's crap. Or I can get the same level of crappiness for no money.
The areas in which software is crap can vary.
It can have a crappy design: non-intuitive workflows. Tabs that say Edit but that don't let you edit anything, Software that says it's OK when it has just coughed a fatal error. Software that doesn't save your work unless you explicitly tell it to.
It can have bugs. Functions that do not work. Ones that produce the wrong result. Crashes, lock-ups, windows that freeze.
It can have a crap-load of dependencies. Yes you've bought (or downloaded) product X - but didn't you know? you also need Y and Z - oh: and it only (or doesn't) work on Debian, sorreeee we thought you'd know that!
It can have crappy documentation. Stuff that was written for the beta version, when the author was young and idealistic. But was never updated. Software that was written as if it was the answers to a Bachelor's degree final exam, rather than for someone who has no clue what they're doing.
It can be slow. So slow you think it's crashed, when all it's doing is initialising. Software that tries to access a far-distant website and then retries and retries again and is still retrying when the heat-death of the universe is looming.
It can be stuffed full of advertising. Whether "free" or paid for, Popups are the very last thing I ever want to see. Although the ones that "pop up" behind some other window, or minimised, are even worse.
But even free (zero money) software has a cost. It takes time to install. It takes more time to customise. It takes time to learn to use. It takes time to search the internet for answers to problems ... and even more time to search for answers that work.
And for commercial software there is the added disincentive that apart from all the time you have to invest, you have to pay extra to even access the software to find out how crap it is.