Elevation of a point on Earth
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.