Spy Sunglasses

I was chatting with my friend JT about spies.  Such an exciting, mysteriously intriguing topic!   

JT said she was at Costco and saw "Spy Sunglasses".  She started to wonder what would make a pair of sunglasses specifically spy glasses? 

 

Then she realized "Spy" was the brand name!

 

I was hoping for lasers or x-ray vision.