Why are bacterial cultures necessary?
With only a microscope, you might be able to find the bacteria, but how do you know what kind of bacterium it is? 

Taking Listeria, under a microscope, it looks like a small rod. Well, so do lots of other bacteria. If you only see that one bacterium, you can't take it and do any further tests with it, you can only see it. 

So we do cultures, where bacteria multiply and form colonies. Whether they even grow under these conditions already tells you a lot - a lot of bacterial species won't grow colonies on The type of colony is another factor in determining what bacterium we are looking at - does it have fuzzy or clear edges, what color is it, is it "shiny"? 

And then, with that culture, further tests can be done, like what chemicals the bacterium in this colony can break down, what they produce, whether they can be stained with certain dyes, etc. Many times this is needed because bacteria look very much alike (round or rods). 

For Listeria specifically, there are special plates used as a growth medium that has certain chemicals added that makes their colonies appear red-violet. Other bacterial species won't show this color even if they do grow on the plates.