What are steam gift cards used for?

Faq, Steam

Steam gift cards are used on Steam to purchase games, software, hardware, and other items you can get on Steam. They come in different forms, physical and digital, and can be gotten in retail stores across the US. Digital cards range from $5 to $100, while the physical range from $20 to $100. For Nigerians and Ghanaians who own Steam gift cards, the only way they get to use it is by selling it for cash.

See also How to Check Steam Gift Card Balance