First of all thanks for the feedback! The use case of mountains is really not that obvious..
The mountain tile helps indirectly by neutralizing potentially harmful tiles. My 3 use cases are: "spot a tile where a city could pop up next turn and neutralize it", "stop a city from spreading by neutralizing the forest tiles around it" and "spend 1 cash to neutralize a savanna which would cost you 1 cash per turn". I actually only intended the first case and wanted the player to also have this AHA moment while optimizing their strategies as I did while testing.