I've using d800 on cam flash as trigger set in commander mode controlling my SB910 in remote mode. I'm wondering what benefit would be using a on cam trigger rather then built in cam flash which doesn't contribute to the exposure at all. Any idea?
Just turn the internal flash off then.
The Commander menu has a group for the built in flash. Change the MODE there from TTL or MAN, to instead be "- -" (one of the MODE choices). That turns the internal flash off, and it will not contribute light into your picture.
It will still flash. It is also the Commander (it has two functions, flash and commander), and a commander must flash commands to the remotes, but this occurs BEFORE the shutter opens. Then after than, it can either contribute light into the picture as another TTL or MAN flash, or you can set that groups MODE to be "- -", and it will not contribute light into the picture after shutter opens.
The Command flashing before the shutter opens can cause the human subject to blink however. FV Lock is the work around.
And commander must also flash a weak signal after the shutter opens to trigger the remotes to fire then too. This is same as using any optical slave, with the HUGE exception that the commander trigger is very weak, very near minimum power, very near nothing - where normal optical slaves are triggered by the full working level of some other flash. It can cause a visible highlight in close shiny things (maybe subjects eyeballs). But except for near macro work (or f/1.8 work), it won't affect the lighting - simply too weak. You can add the $12 Nikon SG-3IR panel to prevent that, but it is normally no issue at all (helps minimize blinking too, but the remote TTL units also do preflash, which is not affected by the the SG-3IR (but again, FV Lock is the workaround).
For more, see
Using the Nikon CLS Remote Wireless Flash System
Off subject - The IR part confuses people.
ANY flash puts out both visible and IR light (a strong IR component) ... just how flash tubes work. Photocells are sensitive to both (to either). Handheld flash meters have a filter to pass visible and block IR (because IR does not affect our picture, it should not affect the meter either). But the Nikon remote flash sensors have a IR filter to pass only the IR part (I think the Sun has a weaker IR component than the flash, percentage wise). The SU-800 commander has an IR filter to pass only the IR part (helps prevent subject blinking, but again, the remote TTL units still fire their preflash too, which can cause blinking too). The SG-3IR filter passes only IR (blocks visible light, which might affect our picture or cause blinking).
The IR part is really mostly a confusion to us, we are concerned with the visible component that affects our picture.