What Atticus means is Scout needs to try and see where the other person is coming from, and what influenced them to make that decision. It sort of means to put yourself in that situation, and it makes you think of what you would do in that situation if you were that person. Yes, I do do this because I can not judge a person based on a little thing I know about them, or their appearance, etc. You gotta try and put yourself in that person shoes. I have talked with people and they are much too quick to judge, while I try and see it from that persons point of view to try and get a better understanding. Put yourself in the other persons shoes first.
I think it’s important to know what we believe because it really does affect the treat others, and we may not even realize it all the time. Some people may be more stern to get their exact point across, while other people would talk in a softer, nicer manner. Some people have different opinions on different things, so for example: some people would say stealing is okay if they had no money and needed clothes or food etc, while other people would say no, they need to work for it or find help and not just take things others have worked for. Your belief system can also help you decide your standards and know when to draw the line and things like that.