Spammers have different purpose for doing that, but I will recommended these things.
Report that website to Google for duplicate content.
Block that website in your adsense dashboard, so the earning and fake clicks or any other invalid activity will not count.
Use filter in Google analytics, if you think you can't see
reports very well.
The reason may be various, it's depend on which kind of spammer is, I mean noob or advance.
Some of noob, just send bot traffic in order to block your adsense, or damage your seo, but it will not going to help them. Since you can block and report that kind of things.
Some of noob, just scrape your website content, and drive traffic and sell it to flippa or any other marketplace.
For advance spammer I have no idea, but you should really check their activity. Their main purpose is only benefits. I have seen some of website (normally porn) is blocked in some of country, so some of spammer take that advantages and clone the blocked website in order to get visitors and monetize later from advertising. It can be various reason, since your website is adsense ready, so that might be not problem for you, but you should check their activity regularly.