To see if the address they sent to is real (no bounce) and most importantly, to troll for a reply/unsubscribe. The reply gives certain information to identify you, which is gold/platinum for hackers.
For instance, if you reply from a cell, and are not properly masking certain things, the recipient of your message/reply can geolocate you fairly well, and in some cases, within a few meters. If you reply from your desk, the same place you have data someone might want to get at, unless you do certain things, you can certainly be geolocated reasonably accurately, and more importantly, the route to your computer is decipherable.
I will not go deeper here, but this is stuff any script kiddie knows or can look up, let alone the "professional" hacker entities.
Like the security company who posts their head's SSN as proof of their protection, with proper security, it is not a big deal if someone knows your IP. BTW, long gone are the days when an ISP would actually change your IP often. It is easier for them, less support and all, to essentially give you a static IP, which only changes on certain rare events.
Even the bounce you received is likely a fake, as they don't want you to know they "got" you until they see what they can sell the information for, or until they get what they can.