Key signatures are a sight reading tool. They tell you what key your in, and then you just look at the staff. Each line or space is a note within the scale of the key your in. The flats or sharps carry over, so that you don't have to write them everytime you play a sharp or flat note depending on the key. Which could be messy if you write in a key with alot of sharps or flats. Accidentals on the score then become notes outside the key. There alot more reasons why it's like this, but in short it's for convience. If your not learning to read music, it has little value. What you should look at though is the circle of fifths. It helps with modulation and writting chords progressions like a cycle five.