sealer is recommended for many reasons, it helps for base coat to adhere better,and the finish becomes more chip resistant, alot of sealers on the market are tintable and can save alot of money and time ,(one coat of tinted sealer and two coats of base is usually enough to get proper coverage versus 4-5 coats of base, you won't have to waste expensive paint just to get coverage),less chance of die-back, since there is less material on the car,
painting without sealer can be done and there are plenty of shops that don't use sealers, but the job just won't be the same, sometimes prep scratches will show through the clear and when the paint gets chipped,most of the time primer color will show,but with sealer,under there,usually only the clear will chip exposing the basecoat,
hope that helps understanding the purpose of sealer