An interface is simply an external (most often) sound card with features that make it perfect for recording and mixing music. The better ones use ASIO as a driver and that comes on a disk with the interface or are available from the manufacturers web site.

Using one and setting it up to be the default sound card for your recording system in your DAW makes your system act like it should act.... with near perfect echo back and extremely low latency for synths.

Using one will reduce that latency (delay) you hear to levels that you can work with. I still hear a slight delay or latency when I work with the amp sims like Amplitube. There's an awful lot of processing that has to be applied to the clean guitar signal to get it sounding like a stack of Fender Twins or whatever else AND get that signal back to the speakers quickly enough to still be in sync with the source material. If that can be done in less than 8 to 10 milliseconds, it sounds like it's in sync well enough for rock and roll.

Those little cables can't make that round trip that quickly so you end up with stuff so badly out of sync, that for the most part it's totally useless to someone trying to record to the music.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.