Supergirl's finally, when Kara goes to be Kara without the glasses, is so powerful because throughout all the seasons of Supergirl, we've been told how important it is to keep Supergirl's identity a secret.
In season 3 in episode 9 where Supergirl lost the battle against Regine and went into comma, the whole point of this episode was that she had to go back to being Kara, Kara with the glasses that Alex loves so much, and this is what get her out of her comma prison.
The whole episode is without her glasses, and only at the end she realizes that she is missing the glasses -there is a close-up on the glasses she takes them, puts them on, and manages to come out of the coma as she goes back to being Kara.
In season 5, when Kara tries to tell Lena in several ways that she is Supergirl, everything always gets complicated and ends badly for both Kara and her friends. Then in season 6 all these things are forgotten and Kara wants to be Kara without glasses because at the beginning when she saved Alex in the plane she was afraid to be herself- so says the Gottenlant -and that's why she didn't pass the test.
Only at the end of the season does she understand why she didn't get through it and only thanks to Lena she takes off her glasses and becomes Kara without glasses.
When this moment happens with Lena, specifically with her and not with Alex, it says so much about their relationship.
With Alex, Kara had to hide behind the glasses she wore, it's true that they helped her take control of her powers, but that's how she actually hid herself, her true self from the world, Kara Zor El from Krypton.
Now, with Alex, she is Kara Danvers And when Lena arrives something happens to Kara, something happens in her relationship with Lena that allows her to be with all parts of herself, even if it took her a little time to understand it or to tell it.
When Lena tells Kara that she doesn't trust anyone and doesn't open up to anyone "but So I met you and you took off my armor with your warmth and seriousness…” It worked both ways because Kara also ends up taking off her armor and she can finally be herself and not behind the glasses.