Its main use is for almost circular orbits as perturbing (non-Keplarian) forces on an actual orbit will cause the osculating eccentricity vector to change continuously as opposed to the parameters eccentricity and argument of perigee for which eccentricity zero (circular orbit) corresponds to a singularity.
Its main use is in the analysis of almost circular orbits, as perturbing (non-Keplerian) forces on an actual orbit will cause the osculating eccentricity vector to change continuously. For the eccentricity and argument of periapsis parameters, eccentricity zero (circular orbit) corresponds to a singularity.