And if you use an iPad, you don’t even have a choice.
An interface is only required when you are recording and making mixing decisions."
RME
Reason one is connectivity.

Universal Audio
But If you have an audio interface, you’re free to plug in several analog mics.
“Audio interfaces are essential for anything related to music production.
They offer higher audio quality than the built-in audio interface of Mac/iPhone devices.

RME
And then there’s the audio quality.
Because an audio interface is purpose-built for its job, it usually sounds better than your built-in connections.
USB Audio Interfaces for Home Use
But what about for home use?

Universal Audio
Is there any point in bothering with an audio interface if you’re not podcasting or music-making?
Sure, but not necessarily for sound-quality reasons because Macs already have great built-in audio outputs.