It's a combination of things. For starters, I generally feel that openness in terms of computing, and technology in the more general sense, is a Good Thing for the world at large. The more things are open source and unencumbered by various legal mechanisms (patents, whatever), the more people who can use those tools, build on them, innovate, and participate in the further advancement of technology. Think "virtuous circle".

Beyond that, when tools are Open Source it makes life easier and takes out friction for us as individuals. I hate the idea of ordering up a fancy new chip, getting excited about it, and then finding out that I have to pay a zillion dollars to download and use the programming tools. Or I get the tools and find out that there's some weird "field of use" limitation in the Terms and Conditions. Or worse, I lose my last copy of the software, the manufacturer goes out of business, and then later I find a few of those chips in my parts box and realize I will never be able to use them because I no longer have access to the software. Or maybe I still have the software, but it has a bug that nobody can fix because nobody has the source code.

That we still have to use some chips that are proprietary to some extent is something I accept out of pragmatism, but by the same token, I support efforts like OpenCores, RISC-V, etc. that may eventually lead us to reasonably usable CPU's that don't need binary blobs and aren't full of undocumented features, etc. At least one can hope.