Okay, so you have two questions. I'll answer both.
Yes, I do keep my word. Whenever I tell someone I'm going to do something, I do it. If I make a plan with someone and someone else asks me to do something that interferes with the first commitment, I politely tell the second person that I'm already commited to something else.
As for your second question, why I feel it's important? Two reasons...
Keeping my word is one of the few things in life that I have control over. And I feel really good that people in my life know when I say something is going to happen, it will indeed happen. I never take that away from them.
There's another question you didn't ask... How do I keep my word?
Easy. I never make a commitment when I have any doubt about keeping it.
There are only extreme cases where commitments have to be broken, or plans get changed. Such as a death in the family. Or weather conditions. Or flight delays. These are things that should always be understood and accepted.