Seat allocation is the main benefit of on-line check-in. Using BA as an example, I am intrigued at how it works. 24 hours before t/o it looks like they do an auto-assignment. This appears to give weight to pax status (e.g. club member).
I changed me and Mrs HKPAX seat allocation to reserve window and aisle seats for the two of us leaving the middle seat empty, i.e. so no-one would choose it given better options. This worked, and in a nearly full plane we had the space between us empty. I wonder if this was allowed by BA's system because we have different family names?