Because you want the correct amount of yeast Cells BEFORE you pitch into the five gallons.
The yeast are going to start working really fast, and they're going to try to chew through 5 gallons whether they have enough yeast to do the job or not, and therefore they will be stressed out and produce off flavors.
By growing them in a smaller amount, they will not be stressed out.
Also, The biggest reason I suggest folks make a starter is if you make one you'll have peace of mind. It's especially important if you have questionable situation happenning with your yeast, like not being sure the yeast arrived healthy.
And you won't be starting an "is my yeast dead" thread in a couple of days.
Making a starter first insures that your yeast is still alive and viable before you dump it in your beer. You will be less likely to start one of those "is my yeast dead?" threads that are on here every day.
You will also ensure that you have enough yeast usually the tubes and smack packs are a lot less yeast that you really should use for healthy fermentation.
Making a starter also usually means your beer will take off sooner, because the first thing that the little buggers do in the presence of wort (whether in a flask or in a fermenter) is have an orgy to reproduce enough cells to do the job...So it won't take such a long time in the fermenter since they started doing it in the flask.
So making a starter proves your yeast is still healthy, allows you to grow enough yeast to do the job, cuts down on lag time, and ensures that you will not get off flavors or stuck ferementations from stressed out yeast.