KevMc
The lower left ad is reserved mainly (for now, at least) special promotions/upcoming events/partner channels available on Roku. They used to be Roku and Roku Channel-exclusive promotions, but have expanded a bit.
For those bothered by this extra ad, they do come and go and are not always there. They will show for a few days or so and then disappear. They are not yet a constant fixture.
Truthfully, I think the more you use Roku, the less you will even notice the ads since you are only on the Home Screen to find your channels/apps. For me, I actually do find some some of the content advertised as helpful as I probably would not have it discovered it otherwise. Now if they start using these ad spaces to start hawking laundry detergent, cars, and products (other than streaming related), then I would likely view them differently.