I do wonder what a brand would actually get out of it.

If you're already someone with a big following in the relevant target audience, then sure, giving you free stuff to test on your trip might make sense - the own-cost of a piece of kit could certainly be lower than the cost of a month's worth of banner ads on a specialist website. But this assumes that you are already doing a lot of documentation and presentation of your trip. If you enjoy doing that anyway, great! If you don't, spending so much of your time on the road setting up various tripod shots, processing photo and video, etc., could be a detriment to the experience.

Unless you happen to live within spitting distance of Touratech Corporate Headquarters, the likely sponsorships available to you will be local companies - the local KTM dealer and the local outdoor goods store, rather than the bike manufacturer and the kit manufacturer. For the local retailer, what's the advantage of having their logo on a bike traveling through places where they don't have an audience?

The only kind of sponsorship that makes sense to me is prototype testing. There's a guy here in Estonia who is making panniers out of HDPE, combining the advantages of soft and hard luggage, and there is an Estonian couple going around the world right now with his prototype cases. That makes all kinds of sense. They give him product feedback, and they tell other adventurers on the road where to order a set like that.