Every time the DeLorean travels in time, it appears in the same place as the place it left. But that's the same place relative to the Earth's surface. In space-time terms, that point is constantly moving: the Earth rotates on its axis at a speed of one revolution per day, and moves around the Sun at a speed of one revolution per year (a roughly circular orbit with a radius of 93 million miles). The Sun is orbiting around the centre of our galaxy, pulling its planets with it, and our galaxy is moving relative to all other galaxies.
So if the DeLorean does not move in space in absolute terms, then a time-jump of even a few minutes would place it outside the Earth's atmosphere as the planet would have moved a few miles in that time. Therefore, to prevent this, the car's computer has to calculate the precise movement of the Earth (and Hill Valley) over the required interval travelled.
A computer powerful enough to do that kind of calculation probably couldn't have fitted into a car in 1985. So where did Doc get it from?
If he could calculate the spatial displacement to remain in the same place on the Earth's surface that accurately, why couldn't he adjust those calculations to make the DeLorean appear wherever he wanted?