To give one legitimate use:

One of the clients I work with is a university. Staff are forced to set-up 2FA.

We received numerous support calls, particularly from users of Safari, who would find themselves accidentally in incognito/private browsing mode - and then complain that the "remember my device" functionality (which relies on a cookie) didn't work.

We solved this with a visible warning to users who are in incognito mode to remind them that they will need to provide a new code each time they login.

News websites want to limit the amount of articles one can read.

Buying airplane tickets.

For those who don’t understand this one, airlines will sometimes artificially inflate tickets faster for people who visit their site multiple times to create a sense of urgency. I always shop for airline tickets in incognito and only log in at checkout.