I assume that, once again, this probably depends on your field and country.
In the Netherlands, apparently, it is required to leave a large quantity (>100) with your university. Also, an ISBN will be assigned, according to my contract. This should mean that anyone could quite easily order a copy. I don't want to know the costs of such an order though.
Sometimes you can also find them on Google Scholar. Although I'm not sure how i will have to proceed to have mine appear there (in years), I like to read them. They usually are well written and give a very good overview of the field in a concise manner. Reading papers to achieve that kind of overview usually takes a lot longer.
This would be my answer to the title question. A well written overview of the work, done during your PhD, in relation to what is known in the field.