He told Lois why
Clark: Your right Lois, I should be wearing a mask. Unfortunately, I've lived my entire life up until this point without needing one. That's why I've been so reluctant. I mean, this is the face that my parents raised. It's the face of the man that you love. And I don't want to deny who I am when I'm out there doing what I was born to do.
Lois: Then why bother with the jacket, Clark, the symbols?
Clark: When I'm out there in the red and blue, and I'm saving people, that's who I really am. And I know this is gonna sound weird, but it's not what I'm called that should define who I am. It's who I am that should define what I'm called. Clark Kent - it's...it's just a name. It's just a word. I am The Blur, and I've always has been.
Lois: Clark, you can't just whip out a new costume at work.
Clark: The Blur is not the disguise, Lois. Clark Kent will be the mask.
Lois: Okay. Okay, but the only way the glasses are ever going to work...
Clark: If I adjust my behavior when I use them. I know. The world needs to believe that Clark Kent is way too normal to be...
Lois: Super.
Clark: And even though we silenced Godfrey, we shutdown Granny's orphanage, and buried Desaad underneath Belle Reve, it's not the last time we've seen Darkseid. But I'll be ready...''(puts on glasses)''...hiding in plain sight.
Lois: So you're willing to dial back the hometown hero and crank up the Average Joe?
Clark: If that what it takes to be the hero the people need, yes, Miss Lane, I am.