Why mailing lists?
Mailing lists are an archaic way of communicating, but they remain an integral part of the web ecosystem. Projects that power services you likely use every day still collaborate over mailing lists. Mailing lists, however, have fallen out of favor for social use, in favor of things like text groups, discord, slack, and so on. However, there are a few reasons to reconsider mailing lists:
Mailing lists are decentralized: no accounts required!
Users do not need accounts to participate in mailing lists. This means that administering a mailing list is easier (you don't have to deal with user accounts), as is participating in one (you don't have to create an account). Nearly everyone has an email account. Many communities depend on for-profit, centralized services like Discord or Facebook for communication, which means some people may not want to consent to giving up their data to these platforms.
Mailing lists are long form
Most of our digital communication has tended towards short form: tweets, "stories", short-form videos, and so on. Mailing lists provide something slower: asynchronous, long-form communication. A "slow web" alternative to the hyper-stimulating web as it exists today.
Mailing lists are stable
Email has been around for decades, and almost certainly will continue to be for decades more. Mailing lists are based on well-supported standards, and you don't have to worry about this or that hosting provider removing your account, going out of business, or becoming defunct in favor of another platform.