so what does Linux have that I need?

That should be the other way around, no? What do you need that Linux has (and Windows doesn’t). Otherwise it’s a case of “solution in search of a problem”. You presently do not seem to have a need as you have mentioned, so ideally you should leave it at that and continue using Windows.

What can motivate me to migrate?

While as I implied above only you can answer that authoritatively for yourself, a few examples of what other people seem to like about Linux might help perhaps -

• “Free as in beer”, so not having to spring for another license if you build another rig
• “Free as in Freedom”, which matters to many but not necessarily everyone
• Better environment for development
• Less susceptibility to malware (not necessarily because of inherent security, but also because Linux is not targeted as much)
• Heavily customizable, at the kernel, desktop environment, other software-level
• Choice of software update mechanisms as well frequency of updates depending on use-case
• Reviving of old computers where Windows would typically struggle to run
• Community participation, though this can be a hit or a miss depending on where you hang out and who you interact with

… and so on.

What is a good Linux to have for a desktop + steam?

There are many, but I generally recommend Linux Mint or Pop! OS for this use-case.