RPG

Branding! Using the OGL you can't use the term "Dungeons & Dragons" (you never could).

Well...you could say 'this is compatible with Dungeons & Dragons' if you do not accept the OGL.

The OGL is not relevant to copyright law

Right - in fact I'm not sure what good it is at all. If you agree to it, you agree to not use:

product and product line names, logos and identifying marks including trade dress; artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs;

...what a mouthful! And all quite subjective.

I've gone with GPL over OGL, as the OGL isn't open. Full post here:

gemini://ttrpgs.com/gen/ogl.gmi