Attending away games means a lot to the fans, but nothing to the club financially. Of course there is the intangible element of better support potentially giving better results, increasing home attendances etc but that is almost impossible to quantify.
For the club, home attendance is...
Loyalty is an emotive concept, and clearly implies far more to a supporter than financial outlay. However clearly the club need a way of quantifying 'loyalty', and financial outlay/regularity of expenditure is as close as you can get to being able to do it.