They want to be appreciated, acknowledged, and seen above almost everything else. If you call up an old acquaintance and start the conversation by asking if they know of any job openings, you will be seen as someone who only values what they can DO for you, instead of who are they ARE to you. Nobody likes being a tool, they like being a friend, and like feeling that they are doing something good for someone that likes them for who they are.

Now, we ALL know that the call out of the blue to catch up often has an ulterior motive but it's polite (and necessary) to at least PRETEND that the ask is not the main reason you called.