Question

Why do we measure the hight of a place/mountain from the sea level​

Answer 1

It’s a very convenient and useful metric, which is relatively consistent across the entire Earth (after accounting for tides, minor regional variations due to variations in Earth’s density, and so forth, and there will always be some variability over time as well). But the main reason is because there is simply no better candidate from which to measure altitude.

It is difficult to say how tall a mountain is from “ground level” because mountains are literally made of the ground. At what point would you start measuring them? The mountain may be a different height if you measure it from the north side than it would be from the south because the point of reference for where the mountain “starts” would be at a different altitude. Ground level is different all over the Earth, so ground level only makes sense as a point of reference for very local objects, such as human structures, trees, etc. It tells you nothing about how high a given point is in relation to the rest of the Earth.