mountain-drive reduces gears.

This means: On a push of a button all existing gears are reduced by a factor of 2.5.
Therefore, it's not just a "nice-to-have" gadget, that gives some additional gears - no - it's more than a doubling of the number of gears, mostly without any overlap!

You can imagine the effect as follows: if you have a 50 tooth chainring installed, on a push of a button this big chainring acts like a 2.5 times smaller one, means a 20 t. chainring!

If you compare this range 50 -> 20 with a triple chainring, you can easily see, that with three chainrings, you never get such a wide range.