Use this expression when something happens that you don't understand, and you expected it to be different.

The easiest way to understand this is through examples. First, imagine that you visit a city that has a reputation for being expensive, but you find out that the hotels there are actually cheaper than in your own city. You can say:

You'd think that the hotels here would be more expensive.

Now imagine that the company you work for is a very large client for another company. However, when you make requests to this other company, they refuse. You can't believe that they're not treating you better. Say this:

You'd think that they would be a little more accommodating, considering how much money we're spending with them.