A lot of people use flat tables for their games, and mountains are not playable with most rules. Solo games with very few miniatures let you put an enormous amount of scenery, and a table with 100-200 trees really feels like a forest. Elevations and mountains allow to break the table in multiple zones, objective marker are more difficult to reach and the overall visual impact modify your approach of a scenario.
Some modular “mountains” where made, but more is better. Sheets of styropor glued together give a nice scenery, and you can put these pieces on top of another to have a more impressive mountain.
While sheets wait for the glue to dry, some rocky elements are sculpted. The flat visual of the mountains is broke by those littles rocks. A coat of white glue to be done tomorrow, then paint.