Having done a day's rallydriving tuition I can offer some knowledge. The reason for sliding sideways into the corner is to give yourself more time to think. If you go in at the perfect angle then there is less chance of you altering your line. Rally corners tend to be blind, so going in sideways & wide allows you to 'open out' the view of the corner before committing to it. Sideways allows you to put the power down early if the corner isn't as tight as you thought, or to wait until you've scrubbed off speed if it's tighter then you thought.