"Is it really necessary?" YES!
"Can't I just use commas instead?" NO!
"When and where do I use it?" The use of colons is really quite simple, and the distinctions they make in your writing will greatly improve your reader's understanding of the point you are trying to make. According to The Chicago Manual of Style, 16th edition, "A colon introduces an element or a series of elements illustrating or amplifying what has preceded the colon." For instance, The "Fresh Catch of the Day" could be ordered in one of three ways: deep fried, pan fried, or broiled.
"The colon should generally convey the sense of 'as follows' or 'the following'." For instance, The order is as follows: your right foot goes forward, your left foot goes to the side, your left foot then slides close to the right, you crouch and clap your hands.
The colon is used when introducing a sequence of sentences. For instance, Kamya faced a conundrum: She could continue her speech as if she intended to delete the entire section. She could try to fit it in at another point in her speech. She could apologize to her audience, saying she had failed to cover a very pertinent point and would like to talk about it even though it should have been covered earlier.
Regarding the spacing after a colon, in typeset matter there should not be more than one space following the colon. When used within a sentence, the first word following a colon should be lowercase (as in the first and second examples above). When a colon introduces two or more sentences (as in the third example above), the first word following it is capitalized.
Colons have lots of other uses, as well, such as in subtitles, indexes, source citations, URLs, mathematical expressions, introducing a speech in dialogue or an extract, at the beginning of a speech or a formal communication where it follows the identification of those being addressed (e,g., Ladies and Gentlemen: or To Whom It May Concern: or Dear Marketing Management Associates:) or when it introduces a direct question. It is also used in plays when introducing a character's dialogue, such as Rachel: I already told you I don't want to go.
Nathan: But it'll be fun. You'll see.
In addition, it can be used to introduce a quotation "either where the syntax of the introductions requires it or to more formally introduce the quotation," like Charles Dickens, author of A Tale of Two Cities, begins his book with a famous line that is often quoted: "It was the best of times. It was the worst of times."
Of course, there are times when a colon should not be used. For example, if the series is the object of a verb, the objects are simply separated with commas (e.g., The organist played the prelude, the hymns, and the recessional. NOT The organist played: the prelude, the hymns, and the recessional.) And a colon should not be used after namely, for example, such as, or other similar expressions.
Now that's not so hard. Don't let those two little vertical dots frighten you. Put them to good use.