Washing your hair daily is extremely damaging. It's like over-washing your fave outfit--the color fades, it gets thinner, and just looks blah. The exact same happens to your hair with daily washing. Color gets stripped out faster, hair looks drab and dull, and looks overall unhealthy.
Washing hair two-three times a week (or every-other day if you have hair that gets greasy quickly), makes a huge difference. Your hair looks much stronger and gorgeous. You end up with fewer split-ends, shiny/healthy hair, and if you color it, color lasts much longer.
Another bonus? If you aren't washing your hair daily, then you also don't need to put as much effort into styling it daily. First of all, that means less heat styling, which is another damaging thing for hair. It's good for your hair to take a break from blowdriers and flat irons. Secondly, it saves you time. On days when you don't wash, you can basically brush it out and apply a styling product.
I know that some hair types do tend to look greasy earlier than others. If that's the case, try using something like Batiste's Dry Shampoo (or any dry shampoo that tickles your fancy). It's like a helping hand between washings that is in no way damaging to hair. Perfecto!
The main message: Keep your hair looking fab--only wash it a few times a week! You'll notice the great results almost instantly.