To prevent fights
This is perhaps the main reason why girls block their WhatsApp last seen. They want to prevent fights with their boyfriends who have become detectives and make all sorts of assumptions, which are usually incorrect, based on the last seen. Hence, they prefer to block it.
She may not be interested in chatting with you
If you are pestering her and she is not interested, she may block her last seen. Perhaps, if you do not know the last time she was online, you will give up and move on. If you continue messaging her, she will not respond.
To stop unnecessary questions
Why didn't you respond immediately? I saw you online. So, to prevent this and other types of questions, she prefers to block it. At least, if you do not see it, you won't question her.
She does not want you to know the last time she was online
Yes, some girls just do because they do not want you to see the last time they were online.