Why would a company use Reddit to discuss work-related topics?

Most companies have some dicussion software such as Slack/Teams, but some enterprise companies do offer forum/wiki software for collaboration (e.g. Atlassian Confluence)

They use Slack or Teams.

There’ll be channels to talk about random stuff, or about fantasy football, or to post pictures of their cats, or whatever.

I’m not sure why this is even a question?

Do you have any particular topics in mind?

Why would they prefer internal forums for something not directly related to their work such as AI or philosophy say?

Because they want to discuss these things amongst the community they spend all day with, as opposed to random people on the internet.

They usually work so there is no time to chat about philosophy.

But if you have a very specific topic around, you can create groups with Teams.

Several years ago I was working for major network hardware vendor, we were using MS Yammer (now it seems to be renamed to Viva Engage) as internal social media. I wasn't heavy user of it, actually I think I have checked it few times during my whole employment there as most of my teammates. I remember it as a place for semiformal interaction, related to company, like bragging about releases and other milestones.