You'll need to go back earlier than 15 years. ConcurrentHashMap and friends were added in 1.6, but String.intern has been in there since the beginning. Since Java shipped with Threads in the standard library, (but no memory model) that meant it would have been very difficult to do concurrent String deduplication yourself. If you agree string deduplication is needed, then String.intern() was a good implementation for a long while.

String.intern() also offers some other benefits for certain use cases. Earlier versions of java did not cache the String hashcode, which meant that to use Strings as hash table keys meant hashing a lot more. But, an interned string can be used in an IdentityHashMap, which was faster for a long portion of Java's early life.

(I worked on a moderately popular Java library that targeted Java 1.5 as the minimum version. It does occasionally come up useful, but only in specific, and increasingly rare circumstances)