When you work outside of your home it's easier to keep up with the trends in society, which is reason number nine.
Most publishers will tell you not to use "slang language" because it dates your work. For example today's preteen will not use the word-groovy like they did when I was that age. And you seldom find a romance heroine who is a housewive in heels and pearls. I realize those are extreme examples but since I began writing for publication in 1991, many society trends have changed not to mention technology. As a writer, unless you are writing historicals, you work needs to keep up with the times.
I am not a gadget person but my son and a co-worker are so they can keep me in the loop on most peoples must haves. I'm a writer and open a dictionary often, however today's young people would probably look up a word through internet access on their cell phones. So even though the publishers frown on language trends, they wouldn't want your teenaged protaginist trudging through the house to grab a dictionary when they would actually look it up through their computer or phone.
Clothing trends are another thing you keep up on when you are working outside of your home. When I started my "day job" career, ladies who worked in offices dressed up. That meant business suits or nice dresses/dress pants with hose and dress shoes. No one had casual Fridays or "jeans" days. As time went on businesses turned to a more casual dress code. Would I have realized this, had I not experienced it on a day to day basis? I don't know. My opinion is it's hard to keep track of social trends or changes if you're not out in the world each and every day.
So another reason to keep the day job is so your writing can reflect the times you live in.