spec scripts are your friend
(Firstly, a definition for the non industry types. 'Spec' is short for speculative, and basically means you're writing something that no-one asked for and no-one is paying for. You simply had the idea and decided to write it.)
When you start out, everything you do is spec, because you are either trying to entice an agent, or you've succeeded and the agent is trying to drum up work, both activities that require proof you can write. Thus, specs.
When I was starting out, I got into the habit of finishing one spec, sending it out into the world and starting the next one immediately. This was a basic ploy to avoid me dwelling on potential failure. Simply start the next thing, focus on that. Don't wait for the phone to ring or the email to ping. Move onto the next thing, which You Will Make Better.
Some writers, once they've started getting work, stop writing specs. Why work for no money when you're getting paid elsewhere? They've proved their chops, surely the films and shows they're helping to make can now act as their demo reel. If they do have a great idea for a new project, they're more likely to limit themselves to a short spec outline. A few pages at most.
I can kind of see the wisdom in this. Why work when you don't have to? There is still no guarantee that your spec will get bought and made, even if you're a successful writer. Why risk wasting weeks, perhaps months honing a script that might come to nothing?
How about because you enjoy writing?
How about because you want to demonstrate a broader range, or an aptitude for different genres that your current back catalogue doesn't show?
I did a quick back-of-an-envelope tally of the specs that my agents possess, most of which they still regularly send out. There are eight, and every single one of them has got me meetings, got me work or made me money.
I'm going to list all eight and roughly describe their genres;
A sci fi fantasy series
A sci-fi horror movie
A hitman comedy movie
A dark superhero movie
A fantasy drama series
A parallel world comedy series
A supernatural comedy series
A thriller series
Now going purely by those broad genre definitions, the range demonstrated doesn't appear massive. But within those eight, I demonstrate every writing skill I possess. There are heartbreaking romances, laugh out loud farces, intricately plotted murders and so on. Pretty much any writing job I go up for, there is a script to fit.
It's no exaggeration to say that I owe my career to those specs. I got the job on Being Human after Toby Whithouse read a spec (number five I think) which proved I could handle drama and wasn't just the joke monkey that Frequently Asked Questions About Time Travel lead him to believe. That itself was another spec. I got a meeting with Steven Moffat after he read one spec, failed to impress him in the meeting, then managed to get another meeting a few years later when his wife, the Sherlock producer Sue Vertue, read another.
There is another reason to write specs which only really comes into focus when you've worked in the industry for a little while:
With a spec, there are no notes, at least at the beginning.
(Another sidebar for the non-industry types. A note is an opinion on how your script needs to change, from someone else involved in it's production: showrunner, director, producer etc. They've spotted what they consider a flaw in your work and want you to correct it.)
When you are writing a spec, just for yourself, just for fun, no-one is telling you the numerous ways it sucks other than the voices in your head. You are free to create in any direction you choose. And that freedom, the sense of pure unfettered creation, is for many the main reason they became writers in the first place.