Even if tuning by ear, when you play with a band or especially are recording, it would be very silly to trust your ear alone because recording hours of takes in an expensive recording studio with many other people's time in a slightly out of tune key would be very disrespectful and wasteful.

Most musicians can tune "relatively" okay, but do not have the ability to tune "absolutely" (perfect pitch).

Here is an example of perfect/absolute pitch that very few musicians have. This is the skill one would need to be able to not use a tuner at all > https://www.youtube.com/watch?v=dw3-k9 (Jimmy Kimmel Puts Charlie Puth’s Perfect Pitch to the Test).

Even if you can tune by ear, it can be faster to use a tuner, especially in a noisy environment. Also, if you listen to harmonics when tuning a guitar by ear, you'll get something like just intonation instead of the equal temperament tuning that you would get with typical tuner apps.