INTRODUCTION
It's been said that you should always capture DI tracks when tracking guitars or bass and there are a lot of reasons for that. Even if you don't plan to ever listen to the tracks, they still can be very much of use to you.
Here are a couple of uses for DI tracks:
VISUAL REFERENCE
Yes, it's that simple - DI tracks provide you with a good visual reference of the performance itself, before the signal is compressed and distorted by the amp and any effects that are in use. The more overdriven an amp is, the less dynamic the signal coming out will be, which might make it almost impossible to spot individual guitar hit-points.
If you take a DI track when recording, you will always be able to spot any problems very quickly from a mile away, which always saves time. It can also be a visual reference for the consistency of the playing or for spotting a dying battery if the guitar or bass happens to have active pickups.
It also helps to check if the strings are on their way out - just reference your current DI tone to the one when you started recording, as it will easily reveal the degradation in clarity and overall tone if there is any.
EASY EDITING
Editing DI tracks rather than recorded amp tracks is just a way more elegant and clean way of going about it, and it also helps you save time, which is always good.
The general idea is this - when editing, put an amp simulator on the di track and edit it as usual, so that you can play back the sounds as if they were going through an amp. The difference between this and editing amp tracks is that it's very easy to cut, slip, stretch and tune one single track, rather than multiple tracks at once, as well as tuning and stretching will sound way better when done on clean DI's.
If you're dealing with bad performances, just use auto-tune or other software to correct the notes, then reamp the end result once it's tuned. That's the only disadvantage of this technique - you will have to reamp the tracks, if you're not sticking with the amp simulators.
CHANGING TONES ON THE FLY
DI tracks are being used more and more as amp simulation software is becoming ever so good and in some ways even better than miking up real amps. The obvious advantage is that you can easily switch things up in the mix, if something's not gelling right - changing the amount of compression, gain and anything else is just a few mouse clicks away.
You can even change cabinet impulses for different parts if there is a need to do so.
This is super helpful for mixing and it can also be an excellent writing tool. Just recording DI tracks for demos or pre-production is a good way to construct songs from scratch and change things up as the writing process progresses. Once you've recorded only an amp track, that's it - in order to change the tones, you will have to play it again.
CREATIVE USES FOR DI TRACKS
DI tracks are often used to supplement amp tones. Arguably the most popular use for di tracks is to create either a stable and fat low end or to create a clanky, grindy grit track for the bass guitar to enhance the amp tones. This allows for a very flexible multi-band control over the different layers of bass tones that can be automated accordingly, to accommodate different parts of an arrangement.
For guitars, a nice way to make overdriven tones more open and spongy would be to blend in a much cleaner tone of the same performance, and a DI track will allow you to do just that.
DI tracks that are shifted an octave up or down and fed into a reverb can create very interesting shimmer effects that can be blended with the main tracks in a subtle way to create a warm and interesting atmosphere. Print the track to an audio file and reverse it to achieve a ghostly effect that could not be possible otherwise.
DI tracks can also be used as a key input for expanders and noise gates to make these processors work more efficiently, because gating an overdriven track would yield far less clean results.
FUTURE - PROOFING
You never know what techniques or gear you'll have in five or ten years from now, so keep your guitar DI tracks in case you'll want to make a remix of an old project of yours. A tone that seems excellent today may disappoint you when compared to what you'll be able to create sometime in the future.
If you're sending the tracks for someone else to mix, it also is important to include DI's in case your tones are not exactly what the mixer would want them to be.
CONCLUSION
See how DI tracks are extremely useful in a wide array of scenarios? It is very hard to come up with a reason not to record a DI alongside the amp tracks. Even if you want a completely “honest” recording with some of the flaws left in, be nice and hide the DI's not throw them out when it comes to editing and mixing - you might just thank yourself later.