Why Write a Screenplay?

For Fun!
Especially if your story is true, and funny. Really funny.  It's surprising how many people have undergone truly outlandish experiences in their lifetimes, or returned from profoundly strange adventures. How unfortunate that so many of these tales are never told beyond a small audience of friends and family.
Or maybe your story is entirely fictional. If writing fiction, such as short stories or novels, is not in your skill set, and you feel strongly that your tale lends itself to a visual rendering (that is, in an audio-visual medium like motion-pictures), then a screenplay or similar document such as a scriptment or treatment, is ideal.   

Regardless of whether your story is entirely true, based on truth, or entirely fictional, "fun" may not really be the force driving you to tell it. There are other kinds of gratification that accrue to you when you take the first step towards connecting with a greater community, participating in a larger humanity, and connecting with your tribe.
For Profit!
If your screenplay sells to a major studio or production company, you could reap an enormous financial windfall. Are the odds of this happening to you good? Not at all. Competition among speculative screenplays is fierce.
However, technology has radically changed the economics of motion picture production. Generally speaking, screenplays that are devoid of complex action sequences, visual effects, and production design, and set in the present-day, often can be financed on an ultra-low budget basis. This means that a full, feature-length theatrical quality motion-picture based upon your screenplay may not be out of reach.  
Moreover, despite the intense competition among screenplays for production and distribution, it is a fact that many, many distributors, exhibiters, and broadcasters are hungry for fresh, original content they can deliver to their customers. It's a creator's world.
For a Second Career.
Again, writing your first screenplay is not likely to generate an immediate source of income, given the competitive nature of the film and television industry.
However, many creative people simply cannot carry on indefinitely in careers that are saturated with drudgery, monotony and meaninglessness. We only live once (as far we know), and so it's crucial we don't neglect the pursuit of our passions.  If you have a distinctive perspective and an authentic voice, your story will resonate with an audience. Writers who find their audience find their success.
To share your unique and powerful experience with others. 
Once you have a screenplay adapted from your story, you have options. First, you can begin to share it with others, including people in the industry such as agents, creative executives, literary managers and producers. These people are not hard to locate, but capturing their interest in your screenplay is something of an art, and to some extent it's 
a matter of connections and networking. 
Second, the barriers to producing a theatrical-quality film, television or web production are nothing like they once were. With a completed screenplay, you can take the next major step towards production. This might mean shooting a short video excerpt from your screenplay, or a promotional clip like a "sizzle video," "teaser" or "trailer." Instead, you might choose to create a budget for your project and then raise the production financing yourself. Nothing prevents you from independently producing a film based on your screenplay except a lack of money and advice from experienced consultants. 21st Century Foy can help.
Putting your story in screenplay form may be the first simple step towards a kind of self-actualization that will renew, rejuvenate and reinspire your life.