The idea would be that for a web app, posting or fetching little bits of JSON every time an action is taken would be less intense than getting a whole page every time. Of course, as you have experienced, that's not helpful if the initial page load is too heavy. Also it's not too helpful for sites that have content to read, rather than actions to take.

ps. "citation needed" is kind of obnoxious when it's something you can think through for yourself.