Proper fit (read: the seal) is necessary for an N95 mask to do its job. So much so that 3M put out a bulletin for US customers cautioning them that fit testing

Proper fit (read: the seal) is necessary for an N95 mask to do its job. So much so that 3M put out a bulletin for US customers cautioning them that fit testing is required when using KN95 masks, even if you are already fit-tested for their N95s, because the KN95 masks are designed for a flatter Asian-featured face.