Indeed, fans are often incorporated with PC casings. These fans are intended to aid in cooling and airflow within the computer casing, which is crucial for maintaining the proper performance and ideal temperatures of the components. Depending on the design and intended purpose of the case, different numbers and sizes of fans may be provided.
It's a good idea to look at the product details before buying a PC case to find out whether and how many fans are included. A few fans may be included in certain situations, while more fans may be included in others—particularly those made for gaming or high-performance setups—to improve cooling.
In the event that the case you are interested in doesn't have enough fans for your needs, you can always buy more fans and place them in the case's designated fan mounting spots. Simply confirm that the fan sizes are appropriate for the case and that the fan mounts are there on the case.