Re: $2500 interfaces:
Actual improved performance,
Perceived better performance (i.e.: Emperor's new clothes syndrome),
Features or specifications you really need,
Features or specifications you think
you need because artist, engineer, or producer X said that's what he/she used on his/her last Grammy winner.
Appearance: It looks good in the equipment rack even if you don't know what half the knobs, buttons and lights are for,
Ego massage, (I only use _________ equipment in my studio!) [insert high-priced brand name],
and bragging rights (Mine cost more than yours so obviously it's better).
Or possibly all of the above because we're Gearslutz and can't help ourselves!