It is to add gain in many cases .. Keyboards usually have a -10 or even -20dB level outputs ... when you plug them into a +4dB line input, you may have to push the output of the keyboard to the max to get a decent level on the recorder, causing slight clipping as the outputs on the keyboard run out of headroom.

Going into a nice pre amp can add a sonic signature as well ... breathing a bit of life into it or making it fatter or warmer sounding .. The bottom line is, do what sounds best to you. You don't have to do it if yopu don't want to.

Not really about the level as much as it is about the sound and unique tone that a great DI, mic pre, eq, comp, channel strip or any other outboard gear can offer in terms of tone processing.

sometimes when you record a keyboard it doesnt sound the same as listening back through the headphone output on the keyboard,
which is dissapointing, synths sometimes lose that "sparkle" and imaging once they are recorded, I understand that this has a lot to do with the studio, gear, etc, but you may want to check out the BBE sonic maximizer, it sort of gives the synths their "sparkle" and edge back.

I have a mate who uses one with his Roland Mc909 and it sounds great.

If you have the triton "studio" get the digital card with word clock for it, sounds great when hooked up to a good clock source.

I know you were asking about dI's and levels but i thought this info may be handy for ya.

Sammyg

thankyou to all for your replys. They are greatly appreciated. I am tracking all my synths and such through a UA 61, wic Im asuming is okay from what Ive read here (sounds okay to me). At least it sounds better than going through my digi002 pre amps.