A set up consists of a single instrument that is guitar and a recording software like pro tools along with amplitube as the guitar effects.
Is there a difference between using the internal mac sound card and an audio interface in this simple setup?
Internal soundcards may give you accurate A/D conversion but get let down by compromises in the audio chip. There can be noise from the surrounding electronics, and the chip itself is optimised, often, to handle a partially amplified domestic capsule mic plugged straight in - so it's neither a proper high-level line-in nor a low noise mic-in.
Going straight into a Mac desktop, I found it possible to achieve pro standard recording, with low noise and distortion, but only after putting in mains ground-lift and a ground spike to the external audio chain, an audio isolation transformer, filters to remove 20kHz hash from a solar array... and careful gain staging. It then became possible to handle straight audio from a preamp.
It worked very well, but with hindsight, it would have been wise to go to a nice little front end box such as the Scarlett 2i2 - a favourite among my narrator colleagues.