As an early career researcher, I am curious to know the advantages of being an external or internal examiner for a thesis. How does it help you in your career? Since you are not being paid for the time you put into the task, I believe there has to be some other good reasons why you should do it. Does it count as service (to the research community) when you apply for tenure? Or, is it a selfless job?
Many things academics do do not help them for their career. Many academics believe in expanding knowledge -- examining a thesis is arguably a way to do so.
For other academics, the reason may be that they are asked by their close collegues and it would damage their relationsship to say no (the collegues could decline collaboration or decline to examing the thesis of your students).
Or another reason to do this is that probably the academics had some thesis examiner themselves and they do this to pay forward.
One advantage I could think of: Depending on the involved universities, you might get invited to examine the thesis defense and thus get to stay and collaborate with their institution.