But I digress, the question is, why would someone hack your 'thing'?
Reason 1: Because it is there
Someone might not be targeting your thing or even your class of thing when they take control. Your thing might be share vulnerabilities with someone that is being targeted. For example the underlying operating system of the device or hosted applications might share vulnerabilities that are common in web servers running on the internet. Many devices would provide a web interface to manage them, and to do this they are likely to use commonly available webservers, eg ngix or apache.
However simply being available on the internet makes something a target. Someone scanning the internet for something interesting to break into would not necessarily know that the device responding on port 80 is a thermostat rather than a webserver.
Reason 2: For the capabilities
In many ways a thing is mostly a less powerful computer with extra capabilities. It's unlikely that someone would hack your thing for the computing power (although people have produced bincoin mining malware for android). However your thing is every bit as capable as a computer in every other way. It could be used to gain a foothold in your network. It could be used as a spam relay. It could be used in DDoS attacks. It could be used to host malware. The list goes on...
Your thing is generally different though, as it is a computer + something. That something could provide a rich set of capabilities that computers don't currently have. For example, a Nest has sensors to identify whether someone is home or not. Imagine if someone were able to hack into your Nest, prior to breaking into your house to check if you are home. Cameras on 'things' could well be used in the same way that RAT trojans are used for voyeurism and blackmail. Consider that you might not have a laptop with a webcam in every room, but you might well put a 'thing' with a camera each room, including the bathroom.
As the internet of things starts to take off, people are going to work out new ways to exploit the new sensors that it brings to the table.
Reason 3: For the data
Not all 'things' have sensors that would be useful in real time but many of them collect some very interesting data. Something like a fitbit can track your activity. We want that information to run our lives, but it is just as interesting to a third party. Things can provide historical data on heart rate, location or any other information that might be collected.
Security is hard and typically is an afterthought. This should get rather interesting when your lightbulbs and door locks get hacked.