That you can transfer your skills to any other company/avenue/whatever. Just imagine going to the next company to get proprietary software B.

They give me a problem to solve, it’s up to me what tools I use.

If i can do it in the same time or faster with my choice of software I will.

You have greater assurance of what the software is doing behind the scenes.

Even if you can’t read code, others who can will; privacy groups (official and hobbyist) likely will.

You can know, even if just through others, sort of what’s going on with the software in a way you simply can’t with proprietary software.

Depending on the company’s policies you may be able to install FOSS software. Outside of the other comments so far, looking at a problem differently may provide different insight.

The downside is that you will likely miss out on integrations provided by the proprietary software. Also, your company’s IT may not appreciate your experimentation. I had one coworker dual boot their computer to Ubuntu, which eventually broke the install. It was shortly after that that the company began locking things down tightly.

You don’t have to hope the company fixes it or is really doing the security checks they’re supposed to.