The explanation I've always heard for the sheet bend is that it is used for tying together differently sized ropes. But honestly, every time I've tied it, it was fiddly to tie and felt sketchy to actually load. For any actual work that is important or possibly dangerous, I would not use a sheet bend. The double fisherman's is far more secure. The flat overhand bend is almost as secure (depending on the rope), and fast to tie. If untying after loading is a priority, you can just tie two bowlines with the loops going into each other - back them up with barrel knots if you expect cyclical loading, as this can cause bowlines to slip.

Everyone I know talks about how easy the butterfly is to untie after loading. But then, they are comparing to an overhand or figure 8 on a bight. If being able to untie after loading is a priority, I use the bowline on a bight.

For anyone reading - please do not try to haul anyone up anything using your newfound bowline skills, unless you are in a very safe situation - like, helping someone walk up a steep hill. Hauling unconscious bodies through the air without appropriate precautions can kill or permanently disable a person. Especially don't do this with cheap Walmart rope that is rated for "trust me bro". And especially don't do this if you don't understand how to preserve your progress, gain mechanical advantage, or lower the victim again safely. Source: rock climber for 15 years, WFR certified, SPRAT certified.