Welcome to VMK! When you play the pirates game, it charges you 21 credits to play. If you don't win, you might come away with 0 credits. However, if you do make a score, you will get credits. And if you win, you will get bonus credits for winning, 100 credits, plus what credits you get from your score. If you play as badly as I do at that game, you might end up spend more credits entering the game then you get back in prize reward. But if you play it pretty good, or better, then you will get more credits in prize reward then spend to play.
Also, you can win a key, which is a magic pin. I believe you get a key for 10 wins at any one level. You then put on the magic pin key, go into the Treasure Room, and use it to open the correct treasure box. You might find treasure inside (sometimes it is empty), like extra credits, a pirate chair, or maze piece.
Lots of players use Pirates as a main way to build up their credits. One way is to play "friendly pirates". Basically you team up with friends. One game your side wins, the next game you let the other side win.