Weird requests for blog guest posts

Over the last few weeks I've been inundated with weird requests from people offering to write guest posts on my blogs. The following is typical of these requests:

I'm getting in touch with you because I'm interested in writing an article
for your blog. I came across your blog post khanya.wordpress.com while
writing for a website on hospitality management - specifically the field's
trend towards sustainability design. During my research, I've found that
incorporating green aspects to hotels, restaurants, and other service
industries has not only contributed to a healthier planet but in some
cases increased revenues.

Please let me know if you'd be interested in an article about both the
design and/or business aspects of sustainable design. Thanks, and I look
forward to hearing from you soon.

To begin with I just deleted them, but then they started sending me reminders, and asking for my response.

So I replied to a couple of these reminders, and asked for a sample of their proposed guest posts, and there was no response to that at all.

Has any one else been getting these offers?

Why would people offer to write a guest post, then remind you of the offer, and then when you take them up on the offer and ask to see the guest post, just not respond?

Are these people who have so much time on their hands that they have nothing better to do than waste other people's time be generating needless correspondence?

Or are they just harvesting e-mail addresses for the purpose of spamming? If that's the case, it seems a lot of trouble to go to -- to insert the name of the blog and all.

Or is it some kind of hidden scam, like the messages I used to get a year or so ago inviting me to a conference in some US city and in Dakar, Senegal?

After getting several of those, I tried to find more about what it was about by sending enquiries asking for further information about these conferences, but none was forthcoming. I assume that the conferences were all bogus, but I wonder why someone would go to the bother of sending out invitations to bogus conferences. They don't seem to be looking for any kind of response, because they never respond to the responses.

Another similar thing seemed to come from one persistent guy who called himself or herself Laure Norman. He said he had important information for me. Eventually I asked what the important information was, and the reply was that the important information was that there was important information. In the end I set up my e-mail program to simply bounce back anything received from that source, and I now see that my ISP (whose addresses this "Laure Norman" impersonated) is now marking it as spam at the server, so it never reaches me.

What puzzles me about all this, though, is what's in it for the people who do it. They're not asking for money, so there is no obvious scam involved. Does it give them some kind of satisfaction to waste other people's time and bandwidth?