Shoes and socks come in pairs because we have two feet. However, some things that are single objects are also described as pairs. You can buy a pair of pants, but it has two legs, not two pants. A pair of glasses has two lenses. A pair of binoculars is similar. You can also use “pair” to describe two of anything. A pair of binoculars is only enough for one person, but a pair of telescopes is enough for two.

足は2本あるので、靴や靴下は2つで1足(ペア)です。ですが、1つのものでもペア「pair」になるものがあります。ズボンは「a pair of pants」なので、ズボンが2つではなく足が2本あるということですね。メガネの「a pair of glasses」はレンズが2つあります。双眼鏡も似ていますね。2つあるものなら何でも「pair」を使うことが出来ます。双眼鏡のペアは1人分ですが、望遠鏡のペアは2人分です。