If a memory block that will not be used still has a reference pointing to it, it cannot be reclaimed by Garbage Collector, and thus it becomes a memory leak. Memory is taken; it cannot be used for other purposes but a developer has no use for it.

On Android, quite often it is possible to see leaked Activities, and this is a major source of memory leaks. It usually happens when there is a global (Application Scope) reference still pointing to an Activity Instance after the Activity has been destroyed (when onDestroy() was called). In such a case, the Activity object cannot be removed from the memory by the GC, and thus all the resources that the Activity was pointing to cannot be freed up as well.

This can easily add up to megabytes of memory, and it gets much worse when the leaking Activity is accessed multiple times; all the instances will be started, and memory will be allocated but never reclaimed.