We are going on holiday with our 3 month old, where it will hopefully be sunny. Given that we won't ever be leaving her in direct sunlight, is there any reason or recommendation for very young children to wear sunglasses? She already has a very nice range of sun hats.
We had Baby Banz too, but my baby didn't like to keep them on.
Evolution has a nice way of protecting baby's eyes even without sunglasses; pupils contract in bright light, preventing excess UV exposure. And babies can close their eyes if it's too bright for them, which they will do. So I wouldn't worry too much about the sunglasses, especially since you will be taking other precautions about sun exposure.
The biggest danger with sunglasses is using cheap ones that do not block 100% of the UV light. Then, the visible light will appear darker so the pupils will not contract much, and the larger pupils will allow lots more UV light to enter the eye. That's often how eyes get damaged.
Your child is still very young and the eyes are continuing to develop. As pointed out in the comments, keeping glasses on her can be a challenge. Whether it's a translucent cloth that's very breathable (I don't know what they are called, but think of a sunshade type cloth), sunglasses or something else, you should absolutely protect her eyes.
Keep in mind that even though the eyes and person are in the shade, looking out into a bright area can still be damaging (I have blue eyes, so know this all too well). So, try to keep her facing away from extremely bright areas, too.
You should have some method of shielding your child's eyes. Sunglasses may not be the best method because they are easily removed and your child could poke themselves with them.
Even outside of direct sunlight you should still be aware that the light level that you can handle might be too much for her.
Some form of shade on whatever she is carried in is probably the best method. Also make sure she is not too hot/cold and at 3 months you may want to avoid a lot of people.