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It doesn't. You, for instance, accidentally book the same flight twice, or your company's travel person does.

Frequent-flyer perks.

Given a certain level with the airline, United for example blocks the seat right next to you to offer you extra space (this can be overruled if the plane gets too packed). Thus, one passenger may be linked to more than one seat.

Some people don't fit in a single seat?

Wouldn't that be better accomplished with two passengers with the same name? You aren't getting that second seat for free! :-)

What happens if the two seats aren't adjacent? :O

Then at least that passenger wouldn't be much more uncomfortable than he'd be under the present system.