Is it usual for finished PhD students to send out physical copies of their thesis to unknown persons? Some time ago, a recently-graduated PhD has sent me a physical (book) copy of his dissertation. The graduate works in a somewhat-related field, but is personally entirely unknown to me (I have some loose connections to the advisor, though). I was reasonably confused by this - I personally have never heard of a custom of sending physical theses to anybody besides maybe parents. I was 100% convinced that I received the dissertation in error (also because the mailing was addressed to my name, but using a wrong department name).

Last week I by chance got hold of the graduate and told him that he sent me his thesis by accident, and asked whether he wants to have it back. He seemed confused and a little bit annoyed that I wanted to give his thesis back - it turns out he actually sent me the book on purpose, assuming that I would be interested in his work. He told me that he thought it is customary to send a finished PhD thesis to people that he thought might profit from its results.

So, is this a thing, at least in some fields? If so, why not just send the core papers or a link to a digital version of the thesis? Mailing out printed copies seems extremely expensive, and also (at least for me) very unlikely to result in anything else than me having another book gathering dust in my office shelf. I am honestly very unlikely to read an entire thesis, especially if I only have it in a dead-tree book version.
This is not uncommon in the country where I did my PhD (the Netherlands). There you have to print a reasonably large amount of hardcopies anyway, typically people get around 200-300 copies in my field. It is not uncommon to send some copies to researchers that you genuinely believe may be interested in the work, usually people that you have been in contact with before, or are in contact with your advisor.

The cost of the thesis printing and mailing is generally reimbursed by the university (of course, all within reasonable limits). In countries where printing the thesis is not so common, I can imagine that the practice of mailing around copies is not common.