It shows your partner that you love and care for them. It shows them what you mean to them, and how much. Plus it makes your partner feel good, and if your like me, seeing how happy you make someone will make you feel good as well.. Though of course you don't want to say it all the time or It'll loose it's touch.
I think i love you are very important because it changes the relationship between two people. When someone says i love you then end up getting married. Also there is the i love you that you say to ur mom and dad and your friends and then there is the i love you you say to the one true love in your life