The Spring Security PasswordEncoder interface exists to make it easy to safely encode passwords for storage in a database. Hashing the password using a secure algorithm with a heavy work factor will slow down an attacker even if they compromise the password database.
Since the interface was introduced, security recommendations have changed as CPUs / GPUs become more powerful and as vulnerabilities are discovered in legacy algorithms. The original StandardPasswordEncoder is now deprecated as the SHA-256 algorithm is considered insecure. Spring offers more secure implementations based on bcrypt, PBKDF2 and Argon2.
However, Spring no longer ties you to a single algorithm. The new DelegatingPasswordEncoder provides support for multiple PasswordEncoder implementations, many of which are available in Spring Boot applications with default configuration. This makes it possible to select an algorithm at run time and to have a database containing password hashes with different algorithms.