If you know someone who is ill, it will help you understand why they act the way they do. If you're ill yourself, it provides a useful way of speaking about the constraints of your daily life. This is particularly helpful for those of us with physical disabilities that aren't immediately obvious. If you look reasonably normal (and many of us with long-term illnesses pride ourselves on living as normally as possible), it is easy for others to forget that you are operating daily within a set limitations: that you're not just being lazy when you walk so slowly, or when you take two weeks to answer simple emails, or when you don't have the "spoons" to answer the phone after a certain point in the evening, etc..
What do I mean by spoons? Read this excellent article and you'll find out.