If you are an illustrator, you NEED a postcard. Especially if you are going to share your Portfolio at our local conference in September.
If you are a published writer, you NEED a postcard or some other promotional tool akin to it.
As it is incredibly difficult to coordinate everyone's schedules for a pre-conference gathering or workshop, I put together a "webinar" blog all about postcards: what should be on them, how to make them, where to print them, etc. I posted it here.
I tried to gather as many of the postcards from the LA SCBWI Portfolio Showcase this summer to help with this. There are plenty of pictures to help clarify this topic. I hope this helps! If you have any questions, let's discuss in the Facebook group. =)