There are plenty of legitimate reasons for wanting a private browser mode. Maybe you're on a shared device and you don't want to leave history. Maybe you're concerned that you're being watched by shadowy government alphabet organizations (and you are). But let's be honest with ourselves here: the most common reason for using Incognito or private mode is porn. Not that there's anything wrong with that.
On the other hand, it's kind of a pain to switch between standard and private modes in any browser, making you less likely to actually use the privacy/security/monkey-spanking feature.