For me it’s part of the paradox of choice.
When a user arrives at your site, the first thing you’re going to do is present them with a list of choices to drill in further?
I think it’s much more interesting to show them what is popular. What are people talking about right now? That’s the stuff about your forum that is interesting long term.
Of course if the user is coming for something specific they can drill down into a category or tag to find it. But they should know what’s active!
Another upside is it feels much less empty in smaller forums (also: brand new forums!). If you started by creating 20 categories, each with one or two topics, a user is going to feel like nobody uses your forum when they enter each category.