[chrisseaton]

I don't get what's 'in it' for people to have overly complicated forms. Like they have to store all the data and in some cases manually input it (like customs forms.) What's the benefit?

Ideally people would have a single universal identifier and then I'd never need to put my date of birth or tax number on another form again.

[gumby]

Most likely theory is that they actually want to use the data (or once did, and nobody deletes it because they think somebody else must need it).

In the more general case of data collection I believe people collect data simply because they can. I strongly believe that the PII collected by advertisers and others is mostly never used. A liability they take on with no benefit.