There is always a personal reason why you're going to read a book, article or even the cereal box. If you remember that reason when you start, you can focus on getting that from your reading.
If you happen to read a cereal box for the sake of looking what interesting things you can learn, then your purpose is to find something interesting.
If you're reading an article on the internet, you purpose is to learn something, what is it you wish you learn? State that and you state your purpose.
Same for reading a book.
I'm reading this book on..... so that I can ...... When you can make a statement using those words, filling in the blanks... you have your purpose.