Why are offset lead-in arcs used?
Not clear in the image in the question is the aircraft type the offset arc was laid down for. Example above is for the B777-300.

Some gates may have been designed for a certain type.

Based on that, a pilot of a shorter/longer [wheelbase] type can use the offset arc as a guide in turning in before/after it.



Thanks to Ron Beyer and mins for the comments that led to this answer.