First, I've talked about the importance of personal distance in the article of how invading the personal space of another's can be a trivial yet a serious mistake. People are very touchy when it comes to their private space - by respecting that you get credibility and trust from them. By understanding how people divide their space - you can have a better notion of where your presence is welcome, and where it's not, and to act accordingly.
Secondly, on the flip side, by careful observation of the personal distance others keep from you (and the alignment of their body) you can measure the amount of trust and affection they feel towards you.