"Mrs Smith", the librarian is keeping data on who has borrowed a popular book. This is valuable data, as she can make sure it comes back on time.

A day later, 'John' duly returns the book and 'Sarah' borrows it - now the data that said 'John' has borrowed the book becomes irrelevant - the data is out of date!

Date-stamping is important, so that people don't rely on out of date information 

Keeping information up to date is expensive!

Imagine that a busy online travel shop wants to provide the latest and best holiday bargains - it is going to need to pay someone full time to keep the data up to date.

Data based on value judgements (e.g. blue eyes, tall, dark, handsome) may be unreliable - people change the way they look all the time.