I think people investing a lot in wedding photos is less about the hard copy photos for some, but more about living the lifestyle that is sold to people by modern advertising, while grasping at self esteem.
I cant speak for the blokes, but I think for women, it very pushed at them that being 'famous' for no particular reason is very important in modern society and part of the 'herd' behaviour. They are also supposed to spend huge amounts of time and effort and money in trying to look pretty as its still pushed as the most important aspect of existing as a female.
Modern women compete with essentially faked women on cosmetic adverts who are flawless, with photoshoped skin, whitened eyes and teeth, change of eye colour, artificial sculpting of the face in computer programs to make their faces more 'in proportion' so noses are make shorter or longer, eye shape is changed, cheekbones raised, lips made wider and fuller. Standards of impossible beauty normal real life women will never physically be able to meet however hard they try.
On a wedding day its the one shot in life to be near perfect and the centre of attention/approval. Your one shot at a pro photographer shoot, making you look like the impossible beauties, giving you your one day of being famous. Its the one chance at being valid in a society that pushes women to be seen in this role. And the photos give women that forever ro hold on to - a huge shot of self esteem in an advertisers battle the women can never ever win in real life, a war that sets people up to fail cos that sells more products.
I see other reasons for photos too of course - nice memories etc, but feel the above is a big factor for many and molegrips and grums posts caused me to dwell on peoples reasons for such huge outlays on weddings in general.