You have noobitus, a common occurance on here. It's the tendency to stress out needlessly about your beer, and to NOT give the yeast the props that they deserve.
They have been doing this for millions of years, so they are the pros.
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.
Additionally it is better for the yeast to consume and reproduce incrementally rather than just dumping them into the fermenter...The yeast will be less stressed out than if you just dump them in.
Stressed out yeast can lead to a lot of off flavors...maybe even (though rare) the dreaded autolysis....Or the curse of 1.030....getting a stuck fermentation because the yeast have bit the dust.
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.