Apple's Mac systems come with a basic audio controller for playing through the system's internal speakers or through the audio ports to headphones or a stereo system. However, if you would like to use a different controller such as a multi-channel USB interface for recording with GarageBand, Logic, or other use, then you can add it to the system and set it up to be used instead of Apple's controllers.
While useful to have, if you configure your system with multiple interfaces then you might run into an issue where the system could revert to its internal controller after a restart, if your external device loses power or there is some other configuration change to the system.
This is usually not a problem and can be dealt with by going to the Sound system preferences and choosing the device to use for alert sounds, default system output, and default input under the respective tabs for those settings. However, in addition to this approach, the system offers a quicker way to set a specific device for recording or playback.
If you enable the volume control in the menu bar (active by default, but it can also be enabled in the Sound system preferences) then you can access this to change the system's output volume. Additionally, if you hold the Option key when clicking this menu, the options will change from volume settings to a device selection menu where you should see the available audio controllers and be able to choose them. You can also select the option to open the Sound system preferences for more detailed settings options.
Unfortunately there is no option in this menu for accessing the Audio MIDI Setup utility that contains settings for aggregate devices, audio formats, and per-channel settings, but this can be accessed with a quick Spotlight search.