Lets explain....
A Direct Inject box takes an unbalanced input signal on a jack and turns it into a balanced signal ehich is output using an XLR connection. This is NOT a stereo signal; it is still a mono signal but it is now being transmitted down a twin-conducter shielded cable.
Microphones are the most common example of a piece of kit which produces a balanced signal directly.
Using a balanced signal has big advantages for long signal runs and/or low level signals because it is very resistant to external electrical noise when compared to an unbalanced signal (such as that produced by a keyboard or a guitar). This is especially noticeable on low level signals like Microphones and some Guitars.
For a stereo keyboard you would need two direct inject boxes, one for each channel.
But would it make much difference? If you are using decent shielded cables and your cables are less than (say) 10 metres long I'm not sure you'd notice much difference. The conversion of unbalanced to balanced tends to boost the signal so the keyboard would be louder. It could be argued that introducing anouther piece of interconnection between the keyboard and the mixer is just something else to go wrong.
So, speaking personally, if you are not very far from your mixer AND your cables are good quality then I don't think using Direct Injection box(es) will make much difference.
Hope this helps....
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John Allcock