While pursuing a PhD in an experimental field, one usually researches a given topic and conducts experiments that are rather closely related to this topic and one another. I imagine this specialization can be beneficial as one attains expertise in the research field and can work in a timely and focused manner.

However, I can also imagine that this specialization may make changing your area of research more difficult when applying for positions as a post-doc. Therefore, I am wondering about the advantages and disadvantages of researching scientific questions one is interested in, but that are irrelevant to ones PhD thesis on the side.