Querydsl was born out of the need to maintain HQL queries in a typesafe way. Incremental construction of HQL queries requires String concatenation and results in hard to read code. Unsafe references to domain types and properties via plain Strings were another issue with String based HQL construction.
With a changing domain model type-safety brings huge benefits in software development. Domain changes are directly reflected in queries and autocomplete in query construction makes query construction faster and safer.
HQL for Hibernate was the first target language for Querydsl, but nowadays it supports Collections, JDO, JDBC, Lucene, Hibernate Search and RDFBean as backends.