Why is the first request to a Facebook IFrame App Canvas Page a POST request?

I noticed when I set up my first FB app today (an iframe app accessed within facebook) that all the requests made to it via facebook are HTTP POSTs rather than GET requests. Is there any reason for this? What if I wanted to implement HTTP caching?

I'm new to developing on the FB platform, forgive me if this is an obvious question but I've googled and can't find the answer.

Can I post iframe on Facebook?

Adding Iframe App to your Facebook PageGo to your Facebook page and on the search bar enter “Static HTML: iframe tabs” or visit this link https://apps.facebook.com/static_html_plus.

What are iFrames on Facebook?

iFrames allow designers to use HTML, CSS, ASP, JavaScript, Flash and more to create custom Facebook content for their Pages.

Facebook POSTs to the initial page in order to pass along the signed request that includes potentially important information for the application.