Elevation of a point on Earth What purpose is served by taking sea level as the reference point for finding the height of any point on Earth when we all know that measuring the sea level is a very complex task and it differs from place to place and also keeps on changing? Why can't we just take Earth's centre as the reference point for finding the height of any location?
There are many reasons.

First, measuring from sea level is traditional. It started before we had an accurate way of measuring distance from the center of the Earth, and probably before most people knew it was (approximately) a sphere.  But anyone living by the shore could determine sea level to within a few feet.

Another reason is that measuring from sea level tells you useful things like pressure altitude, which are important if you're a pilot or mountain climber.  The polar and equatorial radii differ by about 14 miles (21 km), so depending on your location, a distance of say 6,365 km from the center could be deep underground or halfway to the stratosphere.