The use case here is capturing a network input source (mpeg ts stream) and then using that capture for playback later. If you just use a filesrc your playback will run as fast as the machine can process it unless you've got an element in the pipeline that will limit the speed (such as a real-time audio/video sink). If you're testing transcoding pipelines, that doesn't exist. Since this always seems to bite me, here's the recipe from ndufresne on gstreamer irc:

gst-launch-1.0 filesrc location=test.ts ! tsparse set-timestamps=true ! identity sync=true ! ...

That way your whole .ts capture-file doesn't playback in a flash and your test code can actually process a realistically timed input.