I have been doing some recordings from an OP-1 synth using the line out on the OP-1 connected to the microphone jack on my desktop and then using audacity to record while playing to my headphones. I was told this is a very amateur setup and I should use a dedicated audio interface so I borrowed a friends Scarlet 2i2 and tried recording with that and I really can't tell the difference between them. The only possible advantage I see is the 2i2 lets me record 2 inputs at once but other than that I can't see any reason the dedicated device is better.
What are the advantages to these devices that I am missing? Is there a good reason to buy one over just using the microphone jack?