What is the origin of the idiom "a sight for sore eyes" and when to use it?

The idiom "a sight for sore eyes" originated in the 18th century and has its roots in British English. It was first used to describe something or someone that was comforting or delightful to behold, especially after a long absence or a difficult period. The phrase is used to convey the relief and joy experienced when encountering something pleasing, as if it soothes or heals the eyes that have grown weary or sore.