I know actors have to do that- but writers?
I'm not sure what you mean by "have to." There is no legal or moral requirement to have a domain name of any kind. It's entirely a marketing consideration. If you want a domain name, for whatever reason, buy it.
If you decide that you do want one or more domains, you can buy whatever domain name you like, as long as someone else doesn't already own it. As for what domain name to buy, that again is entirely a marketing consideration. You might want a domain for your name, or your pseudonym, the title of your book, the name of your series of books, something else entirely, or any combination of those. Your choice of domain names depends entirely on your approach marketing. There is no "have to" here.
If you're pretty sure you're going to want a domain name at some time in the future, it may be worth it to snag the name now. The
only reason to do that is to prevent someone else from snapping it up before you do, to make sure it's available when you eventually decide you want to put content there. I'm holding
resistance-as-a-resource.com for a non-fiction book that I may or may not ever finish (can you guess the topic?).
Dale