Some people oppose and reject personality systems because they feel that they cheapen individuality, that everyone's far too special to be categorised in this way.
However, having a vague understanding of personality types can be extremely useful in your interactions with others, not to mention that you'll learn more about yourself and how others may see you, or why you do the things that you do.
Different temperaments react to things in different ways, and understanding how to interact with others in a way that they are receptive to is the key to getting along with them and making them happy.
For example, choleric people expect to prove themselves by being challenged, and they challenge others rather confrontationally because of this.
Other cholerics will respond well, and friendships may form as the two cholerics come to respect eachothers' strength.
However, phlegmatics respond very poorly to being challenged as they've absolutely no desire to 'prove themselves'. They prefer nice, gentle friendliness, and get along best with people who do not threaten them.
If the choleric person was to approach a phlegmatic in they way he'd like to be approached - by challenging them - they'd just get upset and scared, and he'd end up frustrated because he didn't know what he did wrong.
If however he approached them with gentle kindness, approaching them on *their* terms, then they'd be much more likely to respond positively.
Most people will naturally assume that others work just like themselves, on a fundamental level, so that they do things for the same reason that they would, or that they SHOULD do the things that they themselves would enjoy. This can end to rivalries between people purely because of their innate differences.
A CholericSanguine might say to a MelancholicPhlegmatic:
"Take a chill pill, get a life. Get over yourself. Grow a thicker skin. Stop complaining so much."
Who may reply:
"I wish you wouldn't be so aggressive, that you'd care more about the feelings of those you interact with... D:"
They are simply seeing the world from completely different perspectives, each believing that their way is best and criticising those who do not follow it.
Being aware that we are all so different allows us to understand and tolerate the different attitudes of others more easily... hopefully!