These cards are usually used for PS3 & PS4/PS4 Pro systems, but you can also go online from a PC to access the playstation network.
The cost varies depending on what you're trying to get. As long as you have playstation network access (or playstation plus on PS4/PS4 Pro), then you can use it to fund games, theme content, playstation plus subscriptions, & more... All if you have enough on the card to fund the transaction(s).