A too low magnification on the Moon for example, reduces drastically the resolution and the contrast of details, so zooming in, you can select on fly the appropriate field of view to correctly circle the area you want to observe. Idem on nebulae. Placing your zoom at the lowest focal, you are sure to never loose your subject. Then you can gradually zooming in until you reach an optimum field of view, which will gives you the best contrast against the background in order to improve details on its low brightness surface. A discrete eyepiece could never offer you such versatility.
Double stars hunting is the favorite task for a zoom.
The zoom performs also well on bright globular clusters wherein by changing your zoom setting you can gradually resolve more faint stars. An irresistible and funny play. Even if the screw loosens a bit it makes very difficult for the zoom to fall out. The Vixen and Tele-Vue have one. Another interesting and well-thinked detail is the large rubber eyecup that stops parasitic light and cold wind near your eye.