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A queue container in C++ is a type of data structure that mimics a real-world queue. For example, think of people lining up at a ticket counter; the first person to get in line is also the first person to be served. In C++ STL, this is achieved through a FIFO - First-In First-Out approach.

When you work with a queue in your code, you perform operations such as adding an element at the end using push and removing from the front using pop. Visualize this as adding or taking papers from a stack where you can only take from the top. Other operations include checking the first element with front, the last element with back, and checking if the queue is empty with empty. Efficient and intuitive, it's ideal for tasks where order needs to be maintained, such as managing processes in a scheduler or handling customer service requests.

In contrast with the basic queue, the priority_queue container sorts its elements based on a set of rules or a 'priority'. It's similar to being in a hospital emergency room where patients with more serious conditions are treated first, regardless of the order of arrival.

A priority_queue automatically orders its elements so that the 'top' always contains the element of the highest priority, which can be retrieved using the top operation. The default behavior is to use a max heap, where the largest element is the priority, but you can customize this order with a comparator function. This is perfect for simulations and algorithms where processing order is determined by severity, urgency, or any parameter signifying 'importance', such as in a task scheduling system prioritizing critical tasks.

The FIFO - First-In First-Out principle is central to many data structures, including the queue container. It's like getting bread at a bakery; the first loaf baked is the first one sold. Similarly, in C++'s queue, the first element added (enqueued) is the first one to be removed (dequeued).

This straightforward approach is intuitive and prevents any complexity that might arise from reordering or prioritizing elements. It's especially useful in scenarios where timing is key, such as in real-time computing and streaming data, where maintaining the original sequence of data packets is crucial.

Data structures are fundamental in creating efficient and effective programs in C++. As the building blocks of software, they allow us to organize and manipulate data logically. Besides queue and priority_queue, C++ provides a plethora of data structures such as arrays, linked lists, stacks, and trees.

Understanding these structures and when to use them is key to optimizing your code's performance and reliability. From the simplicity of arrays to the dynamism of linked lists, each structure serves a unique purpose, addressing the specific needs of algorithms and applications. Effective use of data structures like queue and priority_queue can result in code that is not only functional but also robust and scalable.