Originally Posted by Kovkov

Derailleurs are a simple but crude way of shifting gears. Bend the chain until it has to go to the other sprocket. They work pretty well in nice weather with friction shifters and 5 or less sprockets on the rear. They make sense in niche applications like racing and mountain biking where weight is believed to be a factor.

For normal everyday riding, commuting etc. IGHs are more suitable. But only Rohloff and SA-AW.