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there's a couple reasons
1) as the engine speed increases air velocity in the port also tends to increase, so where the air is entering the cylinder can change slightly and this leads to different mixing and combustion characteristics -hence you need to adjust DI timing with speed and load
2) as the cam timing changes, so the timing of the arrival of the inlet charge air also changes so again you need to alter the DI timing to meet the new arrival time to give you the best mixing
so assuming that subaru / toyota have done their job and if you were to stick with the stock cam timings then you could also leave the DI timing alone
however, move the cam timing and you need to at least check if you need to move the DI timing also
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