Azure Queue enables messaging between different parts of applications. This is used in the development of highly scalable and flexible applications. Components of applications are often decoupled, to enable independent scalability for the single parts.
Queues are also used as an asynchronous method of communication between components that run on different locations (cloud, on-premises, desktop, mobile). It's also possible to build workflows and asynchronous tasks.
A storage account has no limit on the number of queues, as well as the number of messages these contain, but a single message can only be up to 64 KB in size.