They sponsor it because they use the tech Debian develops. If they have a problem with that tech, this sort of stuff makes for good investments. My company used to sponsor a few opensource projects, well over a decade and a half ago. Why? I wanted to support those projects because we used those products. I also want a patch ASAP when we find a problem, even if outside the normal release channels.
It's also a form of advertisement. We'd happily sponsor a few conventions and I'd even send employees to speak at the events. We weren't advertising for customers so much as we were advertising our employment opportunities.
Heck, I wanted the best of the best. I'd sponsor research projects at a couple of universities and then poach the people who worked on the projects. This is pretty normal behavior and there's nothing sinister or malicious about it.