Flaring a barrier end terminal can aid in maximising the protected area when there is insufficient length before the hazard. It can also improve sightlines out of access points or side roads.
While the flared portion of the barrier can increase the impact angle (and thus the impact severity), MASH testing is up to 25 degrees so the likelihood of vehicles exceeding this is low in most situations. Barrier designers need to weigh up the benefits (eg. greater protected area and better sightlines) against the risk associated with increased impact angle. And of course, this assessment needs to take into account other factors like operating speed.