I’d guess it’s there for better alignment. The detent on the 3T that I have works well, but there is some play introduced because the slider can roll slightly on the head of the switch, giving it some slop.
Is the vibrator a linear system or a rotating one? I remember seeing a casing similar to this one on the S6 and S7 (which gave strong, quiet vibration) but it could just be a circular rotating assembly like on the original Nexus 5 (which was honestly horrible.) I’m hoping it’s the former, as that would be a significant upgrade over the standard barrel motor vibrator on previous Oneplus phones.