Inputs, does removing borders except the bottom one have any benefit?

Until material design, form fields always have a border all around. For me, it's a cultural habit. Users use these borders to identify the field in the page. Google itself, on Gmail, Google Search... Always have border around fields.

However, in the material design fields, there is only one border, at the bottom of the field.

What benefit does removing borders - except the bottom one - have?
What publicly available tests show that is no (bad) impact on users?