By default, the behaviour of macOS upon saving a file is to open a simple dialog window, with only a single drop-down menu showing possible save locations. These locations can vary based on the program settings, your most-used save location, or your last-used save location.
Although this is fine for quickly saving documents to common folders such as Documents or Downloads, it is cumbersome to use the drop-down menu when saving regularly to multiple hard drives and previously unused nested folders. Luckily, there is a way to always show a full file browser in the save dialog for more granular control.