I'm a writer. That probably conjures up an image in your mind - the starving author, alone in a draughty garret in the middle of the night, laboriously hammering away on the keys of an old manual typewriter, half-drunk coffee and cheese sandwiches off to one side, a waste bin overflowing with screwed up manuscripts under the desk, a malcontented cat glaring down from the shelf...
It doesn't have to be like that (except for the cat. There's no getting around the cat). You can write your masterpiece on your own, sure, but you're probably much better off getting some outside input along the way. Even Byron didn't live up to the Byronic Ideal of the writer's garret - what else was the Villa Diodati but the ultimate writers' group? As well as Mary Shelley's Frankenstein, that meeting also produced Polidori's The Vampyre.
Following in the footsteps of Byron and the Shelleys (now there's a band name...) may not result in a work of that calibre, but there are several good reasons for aspiring authors to seek out writers' groups of their own. And you won't have to go as far as Geneva, either...
It's social. No (wo)man is an island. Alone, we starve ourselves of inspiration. Not that anybody is going to steal your ideas if you go to a writers' group - that sort of behaviour is thankfully rarer than hens' teeth - but watching and listening to other people talk about their writing, their ideas, their plots and characters, can sometimes kick-start your own creative processes. And while no (wo)man is an island, the corollary is that together we are archipelagos of enthusiasm (Alasdair Stuart's phrase), which means there's the added bonus that when you say "I write epic fantasy/urban fantasy/hard SF/Wolverine erotica" in a group of writers, people don't draw back for fear of getting geek cooties (normal people are so over-rated).
It broadens the mind. Reading about stuff on the internet is no substitute for actually talking to real people about it. And yes, I am fully aware of the irony in that statement. You'll learn about pacing, about info-dumps and character motivations, and you may even meet published authors in workshops (Open Road Writers in Chesterfield recently had both Rod Duncan and Emma Pass running workshop sessions).
It thickens the skin. Your mum likes it. Does she, really? Or is she just saying that because she's your mum? A good writers' group will deliver constructive criticism. A good writer won't wilt under that criticism. If you have any ambition to submit your work for publication, you'll have to get used to criticism and a face-to-face writers' group is one of the best places to learn that skill.
It strengthens discipline. From a personal point of view, this has been important. Because this group meets twice a month, I've got definite deadlines. Twice a month, I need something I can read out, something that can be critiqued. It gives my writing some much needed impetus.
Those are the most important reasons I've got for joining a writers' group. You may have your own: let us know what they are!