I don't understand why I can't hook my keyboard directly to my computer and record the sounds from my keyboard. Isn't it just midi out of my keyboard and into garageband?
A MIDI cable does not send sound. It sends little messages that say what you are doing. One message is sent when a key goes down and another when you raise the key. If you play a triad chord six messages are sent, a key-down and key-up for each note. What is sent is logically equivalent to sheet music. In fact notice how Garage Band can display what you play as notes on a grand staff. That is because what you are sending is "G4 keys down at X velocity", and not the sound of "G".
The line out jacks send sound all ready to go to a speaker.
The advantage of recording MIDI is you can edit it. Correct wrong notes or fix the tempo. Or later assign the notes to different instrument. MIDI is kind is nice for pactice too. You can see that you played a dotted 1/16 when you meant to play an 1/8 note. I'm not good at time and man a recorded performance shown in music notation shows that in a very clear way