Can anyone explain to me, why so many organizations collect your first and last name (rather than just your full name) on the sign up page?
Because it's helpful to know the first name and the last name, rather than just the full name
Why it can't be done automatically
Without asking, a site cannot tell the difference between the following names:
Why it matters
Sites often want to be able to communicate with customers using different styles, where first and last names are more effective than full names.
- Dear Mr. Morgan, we are pleased to offer you the position of Chief Executive of our Too Big To Fail bank.
- Hi John, it was great seeing you at our sample sale on Saturday. We hope you will return as a customer for future events.
- Welcome back, John!
Being able to use a customer's first or last name can feel more polite or personalized than using a full name:
Dear James Pierpont Morgan, we are pleased to offer.... Hi James Pierpont Morgan, it was great seeing you... Welcome back, James Pierpont Morgan!
Because it is impossible to split later in many cases without input from the user. Eg. when people give 3 words, you don't know if it is a double barreled last name or first name (or if they have included a middle name).
One thing to consider is how first and last names are handled in different cultures. One reason why it's better to only have one field for full name instead of separate fields for first name and last name.