Some times there is the need to upload big files, with more the 1 Gb on webpage forms. Doing this with a traditions HTTP POST request is a recipe for disaster. Most servers are configured with limits that will allow for post variables with that size, feed back to the user is limited giving the false impression that the page is just unresponsive, etc.
One of the solutions is to use javascript to break the file into chunks and send the file over HTTP in pieces.
I am quite found of the Plupload library, that also supports multiple file upload and integrates nicely with php.