ACME takes all those steps that an administrator has to do and makes them automatic. Instead of filling information into a form on the web and following written instructions, the server that needs a certificate can send in its information in a standard form, and get instructions that it can read and follow automatically. In many cases, this allows a server to get a certificate more or less instantly and with no human intervention at all, so that security can be on by default.
Back in 2015, Let’s Encrypt created a new Certificate Authority using an early draft of ACME, which let people start experimenting with the protocol. Through the IETF’s open process, ACME was updated to incorporate feedback from other CAs and users of certificates, and today several CAs have ACME interfaces either in production or in development, including BuyPass, Entrust, DigiCert, and Sectigo. Together, these CAs account for the majority of the certificates used on the Internet; Let’s Encrypt alone uses ACME to issue more than a million certificates per day.