When students ask me “should I really buy a fast lens?” (For beginners, that’s a lens with a low “F-number”, like f/2.8), my answer is “it depends.”
What are you shooting? Landscapes (no need for a fast lens, since you will shoot at f/16 or above) or nightclubs (which need a fast lens for low-light abilities), portraits (which need a fast lens for blurry backgrounds) or sports (which need a fast lens for fast exposures)?
And if you like blurry backgrounds, does it make sense to get a pro lens like an f/2.8, or is my kit f/5.6 lens enough? That’s an easy one to answer. It depends. On whether you like this, taken yesterday during a course at f/5.6:
..or whether you prefer the same shot at f/2.8:
You decide. View them full size to really see the difference.
Know that every stop faster (from 5.6 to 4, or from 4 to 2.8) doubles the lens price. But if you like the blur (“Bokeh”) in the bottom shot, there’s no substitute for fast.
And I did not say expensive – at least not necessarily so: while some lenses like my f/1.4 35mm cost $2,000, an excellent 50mm fixed (“prime”) f/1.8 lens (a “nifty fifty”, which on a crop camera is great portrait lens) can be had for as little as $150 or less.
So yes, low f-numbers make a difference and that’s why photographers are willing to pay lots of money for them. But don’t worry: good lenses keep their value.