I'll give an example of one that I've noticed: the number of web sites that ask for your email address (GOOG ID, YAHOO! ID, etc.) has skyrocketed.

I can come up with no legitimate reason for this other than (1) password reset [other ways to do this], or (2) to remind you that you have an account there, based upon the time of your last visit.

Why does a web site need to know your email address (Google ID, etc.) if all you want to do is...

• download a file (no legit reason whatsoever)
• play a game (no legit reason whatsoever)
• take an IQ test or search a database (no legit reason whatsoever)
• watch a video or view a picture (no legit reason whatsoever)
• read a forum (no legit reason whatsoever)
• post on a forum (mildly legit reason: password reset)
• newsletter (only difference between a newsletter and a blog is that you're more likely to forget about the web site than you are to forget about your email address -- the majority of web sites do not send out newsletters, however, so this can't be the justification)
• post twitter messages or other instant messaging (mildly legit reason: password reset)
• buy something (mildly legit reasons: password reset + giving you a copy of a receipt that they can't delete, as receipts stored on their server can be deleted)

On the other hand, I can think of plenty of very shady reasons for asking for this information:

1. so the NSA, CIA, FBI, etc. can very easily track what you do by reading your email or asking GOOG, etc. what sites you used your GOOG ID at
2. to use the password that you provide for your account in order to get into your email account (most people use the same password for all of their accounts), find all of your other accounts in your inbox, and then get into all of those accounts
3. sell your email address to spammers

These reasons, I believe, are why you are constantly asked to provide your email address. I can come up with no other explanations whatsoever.

Question 1: Can anyone think of any legitimate or illegitimate reasons for asking for someone's email address?

Question 2: What are some other interesting internet trends of the past ~10 years?