Noting that all Cross Country services stop there and presumably SWT services also, does Southampton Airport really generate that amount of usage, or is it used mainly for it's parkway aspect?
If the latter is the case, why do all late night trains stop there as well?
There's definitely far more non-airport traffic, but there's still a healthy number of airport related passengers.
The station is usually quite busy through large parts of the day and attracts passengers from a wide local area and beyond. For example, it's known that people even drive there from areas like Ringwood and the the northern outlying areas of Bournemouth to catch trains to London, because it's easier to get there and park, rather than going into Bournemouth or Christchurch.
For many people living around the Southampton area, Airport Parkway is the station of choice, as it's easier and more convenient to get to than going into the city centre and Southampton Central.
Being situated alongside junction 5 on the M27 and near to the M3/M27 junction also helps draw in passengers from the surrounding towns and villages.
At 1.5 million passengers p.a., it compares quite well with many town stations across the UK. e.g......
Burton-upon-Trent 0.7 million p.a.
Carlisle 1.7 million p.a.
Banbury 1.9 million p.a.
Wigan Wallgate 1.6 million p.a.
Both Wrexham stations combined 0.6 million p.a.
Both Lichfield stations combined 1.5 million p.a.
Halifax 1.8 million p.a.
Taunton 1.2 million p.a.
Penzance 0.5 million p.a.
Lincoln Central 1.6 million p.a.
Macclesfield 1.3 million p.a.
Apart from one or two very early morning trains, all services stop here (SWT express, SWT local and XC).
In comparison, the larger Eastleigh station just a mile or so up the line, only has the locals, stoppers and Portsmouth-Waterloo trains. The Southampton/Bournemouth/Weymouth expresses and XC trains do not stop at Eastleigh.
The Airport Parkway station has also overtaken Eastleigh and has slightly more passengers p.a.
With the large car parks regularly full and overflowing, a few years ago it was decided to build a dedicated multi-story CP in addition to the surface parking. IIRC there are now over 900 car parking spaces there.
As for the airport; it's a fairly busy local airport and has quite a lot of flights, including quite a number of domestic services to northern and Scottish destinations, as well as serving quite a list of international destinations.