The science behind companion planting

I just read your blog from 2008 on companion planting which I found very interesting! It explained why I have so little problems with pests in my combined vegetable/flower and herb garden. How close do these companion plants need to be to each other? I assume the ones which add nutrients to the soil need to be quite close, but what about those that attract beneficials/repel pests? Can they be nearby or in separate raised beds? Within what distance? Thank you very much. This is a wonderful web site.