If it's possible to do a post-doc in the group you did your PhD in, which I assume should not be too hard if you have a good relationship with your advisor, why do researchers often choose to do postdocs at other departments?
If one does a post-doc at another lab, one has to face the cost of adjusting to a new environments and labmates and take the risk of working with an advisor he/she doesn't have good chemistry with. These costs are further augmented in light of how short a post-doc normally is.