I'm not sure whether you're being serious or sarcastic. In the former case, the answer may span from easing the process of review from the outside, hence better the quality of software (e.g. a cybersecurity company would gain from that, given that all major cryptographic algorithms and such are produced by academia in public papers and depend on computational power, not secrecy of algorithms), to satisfying your ethics (I'd rather live in a world where everyone can see with their own eyes if you fulfill you promises about your code, Stallman can teach us a thing or two about this matter). Moreover, the open source community has produced the best software we, as humanity, have created so far: that alone is a good reason to believe in the power of opening your system.
Why should any given software be open-sourced? What is there to be gained by the company?
I'm not sure whether you're being serious or sarcastic. In the former case, the answer may span from easing the process of review from the outside, hence better the quality of software (e.g. a cybersecurity company would gain from that, given that all major cryptographic algorithms and such are produced by academia in public papers and depend on computational power, not secrecy of algorithms), to satisfying your ethics (I'd rather live in a world where everyone can see with their own eyes if you fulfill you promises about your code, Stallman can teach us a thing or two about this matter). Moreover, the open source community has produced the best software we, as humanity, have created so far: that alone is a good reason to believe in the power of opening your system.
Adoption, Community and more
Caveats apply