It will help keep your head from bobbing outside the vehicle as it rolls, preventing your death from a crushed skull.
Edited to add:
"partial ejection" are the magic google words. Here's an example:
"It was found that, in standard impact tests on high containment barriers, partial ejection of the head through the side windows occurs systematically"
Edited to add:
"partial ejection" are the magic google words. Here's an example:
"It was found that, in standard impact tests on high containment barriers, partial ejection of the head through the side windows occurs systematically"
from: https://trid.trb.org/view.aspx?id=750825