Anthologies are an edited, compiled collection of short stories between two covers. Many authors, many stories, many flavors. There are two major reasons writers--of all standings--should write for and read anthologies: to practice (many things, which will discuss) and to sample (these same things).
The first reason writers should write for anthologies is simply to practice the art of the short story. For myself, novels are easy to plot. I have at least 100K words to make the change in the characters and convey a cool story. 10K word? Or worse, 3k words? Yikes! I love the challenge and I've gotten better at it. I used to think (oh, young writer, you poor thing) that anthologies were worthless because no one read them. I'm not sure where the stigma of anthologies comes from (probably pretentious, inexperienced writers). But all the greats wrote short stories. And in our modern-day, the desire for shorter, sensational pieces that can be taken in quickly is HUGE! It will take some time to get new readers into anthologies, but it is also a good tool for new readers: short, complete stories will be more attractive and introduce a reader to an author. The same goes for writers: sample a dozen kinds of stories, writing styles, characters, beats, and arcs--all in one book! What a treasure trove.
The second reason writers should write for anthologies is to test characters, magic systems (for fantasy writers), to help themselves establish a world with more potential, and to explore dangerous themes and topics. I use short stories for this very purpose. Now, anyway. I used to make new worlds for every story and my inspiration ran dry quickly. Now I use short stories to touch on other characters (those side characters you love so very much for example!), test out a magic system, or play with themes. I believe that short stories can deal with heavy themes quite well. The shortness of them makes it very effective. As a writer, you should always be stretching your ability. And there is no harm in reusing ideas. Especially ones that don't make it into large circulation. When you see an anthology call, write a fresh, new story! Use the theme (if there is one) to inspire you and make it your own. Make an entirely new, cool, little unite. Plus, it gives you a chance to try a new workflow. Experiment with outlining or word sprints. Don't just write the story: USE THIS OPPORTUNITY to teach yourself something or try something. It's easier to do it with a short story than a novel. Write voraciously and always. The more different things you write, the better writer you are.
Here is my rule about anthologies (and multiple projects in general): Don't take on too much (the Flex is not worth it) and write for things that are interesting. At first, maybe write for every single project you can find but once you have entered the circle of information and had a few things accepted, be selective. You will write better for projects you like or are interested in.
That's it :)