What's the big deal with Honda SI's?

I have a 2001 Honda Civic EX and I got it smogged today. When the guy was checking out my car he reacted with a big "wow" and started to explain that this was an 'SI'. He was excited to see it and explained to me that it was worth something. Can someone help me understand what makes this car valuable and what would a car like this be worth? It's a salvaged title and I'm not trying to sell it; but, I just want to have an idea of what I have. Thank you for your reply