The purpose is to propagate the yeast, which basically boosts its numbers to take on the fermentation process. If you do comparisons between dry yeast and liquid yeast, you'll see that the cell count in dry yeast is higher, thus negating the need for a starter in most circumstances. Liquid yeast will struggle on some higher gravity beers and may get stressed out due to a lower overall cell count when compared to dry yeast.
I have never made a starter using dry yeast...and I've never had a problem with fermentation. On the other hand, I've had issues pitching liquid yeast without a starter when pitching into a higher gravity beer.
Some dry yeasts will suggest you re-hydrate the yeast in a sterile water prior to pitching. Again, not necessary. I pitched an un-rehydrated dry yeast directly onto the wort and it worked just as smoothly as the same brand/strand when I re-hydrated in sterile water.