There are actually multiple reasons for this.
According to Manga, Sarada's eye condition was explained as an illness as a child, post which she had to wear glasses. I can't find the exact panel, but Narutopedia kind of corroborates this. She has perfect vision while activating the Sharingan, while her vision is no better or worse
During her father's absence from the village while gathering information on Kaguya Ōtsutsuki, she became ill with a high fever and afterwards, began wearing glasses, which unknown to her were a gift from Karin.
Sarada Uchiha: Narutopedia
Sharingan enhances her vision, but only while activated and the normal vision is just like everyone else.
However, the real reason is out-of-universe. It was Design Aesthetic, plain and simple. Kishimoto felt like giving her glasses, so he gave them to her.
Interviewer: How is Sarada? [Or what about Sarada? T/N: In the context of design.]
Kishimoto: Rather than cuteness, I wanted to project a slightly dark but strong hearted sense to Sarada. In my head, she is like a girl version of Sasuke. However, as it would be scary to have a Sasuke-like girl, I gave Sarada elements of Sakura as well. Although spectacled people are commonly given the image of “cute without glasses”, in Sarada’s case I aimed to make her cute even with her glasses on. Her clothes have a sense of Sakura, don’t they?
Kishimoto Interview Excerpt