Minecraft wiki entry for Best_biomes_for_homes, associated text: The mountains biome has very scenic views.
Emerald ores are scattered underground, providing a helpful start for trading with wandering traders if the player cannot find a village. There is also quite a bit of exposed coal ore and stone.
One of the better biomes to farm ice and snow. The farm must be built over y=95, but most mountains reach far beyond that height.
Exposed stone makes it easier for beginning players to obtain it, in turn making it easier to upgrade both gear and building materials.
Lava can be found without having to go down deep into a cavern. Lava pools will commonly spawn on the surface, allowing for easy construction of things that require it (cobblestone generators, nether portals, chicken farms, etc.).
It is very easy to find and harvest coal, which means logs do not need to be used as charcoal.
Zombie villagers spawn here wearing unique clothes and can be cured into snowy villagers.
The biome gives a high vantage point, useful on servers and for PvP.
Iron ore is easy to come by when digging from the tops of mountains.