Who says you have to?
You may however choose to do so as a matter of prudent business practice.
For example, you may simply want to track and identify users who over-use your service
(such as those who download stuff en masse using your data volume),
or who do so for unlawful purposes - anything from downloading infringing copies of TV shows,
to publishing hate blogs, all the way to distributing criminal content.
You might also want to analyse and compare internet a customer's internet use
to their in-store spend.
For example, you might choose to use software to offer a different level of service
to the users who buy one latte, but still sit on your wi-fi all day,
compared to the guys who spend (say) $20 or$50 or even $100 a day,
sometimes over two full meals and numerous coffees.