One use for highly elliptical orbits is being able to linger over some part of the earth for longer periods. https://en.wikipedia.org/wiki/Molniya_orbit https:

One use for highly elliptical orbits is being able to linger over some part of the earth for longer periods. https://en.wikipedia.org/wiki/Molniya_orbit https://en.wikipedia.org/wiki/Molniya_orbit I haven't seen the orbital parameters of the X-37, so it may be in an elliptical orbit for some other reason, such as spying on or interfering with geostationary satellites or who knows what else.