What is purpose of anorm's Pk?

I'm writing Playframework 2.0 application using Scala and Anorm to access db.

Currently I'm using Pk[Long] for id fields and I'm worry about additional get call needed to access actual value. So I start using plain Long for id fields and everything still work perfect.

What Pk is for, and should I use it instead of plain types? Does Pk gives me additional features/benefits over plain types?