Depends on what your intentions are.
There are people who are born with talent dripping from their every pore, who simply take pencil in hand and beautiful images flow from their imagination, straight down their arm and onto the page (or canvas), as if by magic.
Then, there are those of us who simply have an ardent desire to create, who learn painstakingly, by studying art books (both how-to and history), by observing and copying those artists we admire, over and over, until we gradually pick up certain techniques over time by practice, practice, practice...
Both types can only benefit by receiving advice and instruction from those with more experience and knowledge. How much you will benefit will depend on how sincere is your desire to improve.
Whether or not such instruction is "necessary" depends on what you intend to do with your craft. If you're going to be an amateur, creating art for your own satisfaction, then training may not be worth the expense. If you intend to make a living as a professional, in the graphics field, where precision and speed matter, you're not likely to get hired without a degree, and a portfolio.
I never finished college for the same reason I barely graduated high school - I'm great in subjects that appeal to my personal interests, like English and History, but horrible at subjects that bore me, like Mathematics. So, I never got to work as a comic book artist, which was my childhood fantasy. On the other hand, I have worked as a tattoo artist, where a Graphics Design degree wasn't required.
BTW, there are schools that specialize in training artists for Graphic Design degrees, that operate like Trade Schools - a two-year program, without a lot of extra requirements beyond the art courses. However, my understanding is that the curriculum is rather grueling, with no forgiveness for slackers.
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"I don't mean to sound bitter, cynical or cruel - but I am, so that's how it comes out." - Bill Hicks
