Simpler eyepieces provide smaller field of views with less lenses. These eyepieces are ideal for observing smaller objects( planets, double stars, etc) but of course they can be used to observe larger objects as well. Of course eyepieces with larger field of view might provide more pleasant view.
Eyepieces with large field of view provide magnificent views on deepsky objects. They can be used to observe the Moon as well.
The 2" (50.mm) eyepieces can provide much wider field of view at low magnifications. While a 40mm Plossl and a 38mm SWA eyepiece provide similar magnifications, but the latter one gives almost 2 times wider field of view and can show much larger objects as well.