Read/Write locks allow in multiple readers a the same time, but only if no writers are already in. Hence under some usage scenarios we can get better concurrency, because the reader populations can proceed together
The Lock framework is a compatible replacement for synchronization, which offers many features not provided by synchronized, as well as implementations offering better performance under contention. However, the existence of these obvious benefits are not a good enough reason to always prefer ReentrantLock to synchronized. Instead, make the decision on the basis of whether you need the power of ReentrantLock. In the vast majority of cases, you will not -- synchronization works just fine, works on all JVMs, is understood by a wider range of developers, and is less error-prone. Save Lock for when you really need it. In those cases, you'll be glad you have it.