The way the trick works is that someone is trying to sell you something (may be legitimate or a complete scam). They obtain a list of phone numbers (or try them at random), and they spoof their caller ID such that some of the beginning numbers are the same as the number they are calling. In the US, we have 10 digit numbers and they usually set the first 6 numbers to be the same as yours. The reason they do this is so that it appears to be a local call, perhaps even one of your neighbors, and they're hoping you'll be more likely to answer the call compared to some number you don't recognize at all. This is actually helpful to me because now I know whenever I see a call where the first 6 digits are the same as mine, I know for sure it's a spam caller and I can ignore. Even if the caller knows that some people are wising up to this, they still benefit from continuing to do it because then when someone does answer, there is a greater chance that the person could be sold something (as people like me that definitely can not be sold something don't bother answering).
Years ago, when I used to always answer my phone, in one weekend two different people called me from similar numbers to mine, and they asked me why I had called them. I had to explain to them that I didn't, and related to them my explanation above. One time, the robocall screwed up and actually called me from my own number, which I answered and was admittedly confused until I figured out what had happened.
To specifically answer your questions:
What threat models could the spoofer be exploiting?
In most cases there is no threat, just an annoyance. It's possible that you could be targeted and someone could spoof a number of someone you know, and make up a story about them and perhaps persuade you to do something you wouldn't normally do, but this is probably unlikely. I'd just keep it in the back of your mind though that this is possible.
What can I do (beyond changing my number)?
Changing your number most likely won't help, since your number is not being singled out. Millions of robo calls are being made daily and (in the way they do it in the US) whenever someone is called that has the first 6 digits as your number, there is a 1 in 10000 chance that your exact number will be used as the caller ID.
How easy is it to spoof a number in this context?
It is extremely easy to spoof caller ID. One time I received a call from 00001, I answered, and it was my friend testing out a new VOIP system. I asked him about the caller ID and he said, "Oh, I haven't typed in what I want the caller ID to be yet."