A valid credit card number has several fields and each of them has a meaning.
Anyone can make a website with a form and require you to insert valuable and sensitive information which requires you to give up your privacy.
This is a way to protect yourself in such situations.
The other reason we made this are programmers testing ecommerce websites, applications or other software.
They usually need lots of fake data, and this is a very easy way to generate a bunch of valid credit card numbers in a split second.
There's another tool for those times when you need to generate all other kinds of data.