Hi, there a few reasons you might want to feed into the camera, the most common one being you can't be bothered syncing the sound later.
You could use the in built mics on the Tascam which are not only better than the camera mic, but are also stereo, and you'd get a better sounding stereo audio track on the video in camera. Because the Tascam has xlr inputs with 48 volt phantom power, you could also use better quality condenser microphones, and get an even better stereo (or mono if you choose) audio track, and still record it onto the video, so later on you don't have to resync the external recording.
Even if you are using the external recording and resyncing it later, having exactly the same audio on both the recorder and camera means later on when you zoom in on the waveform to match sync, the waveforms will be exactly the same to look at, whereas if you use the camera mic they will look a little different even though it's the same sound.
The other thing that can make it a little trickier with syncing later is how far the camera is from the audio source. Sound travels at roughly 1 foot per millisecond (not exactly, but easiest rounding of number to remember) so if you have a boom or lav mic right by the actor, but the camera is 15 feet away, the camera mic will pick up the sound 15 milliseconds later than the boom/lav. So when you sync to the camera mic, you can be slightly out. 15 milliseconds maybe some people wouldn't notice, but 1 frame at 24 fps is about 41 milliseconds, so if there's 20 feet between the boom and the camera you are talking half a frame out.
Still none of this means you cant just record to the Tascam and sync to the camera later, it just makes things easier.
