the idea of burying treasure is that it is for safekeeping. if you try to carry a bunch of treasure around with you someone will steal it from you. if the treasure is hidden somewhere only you know about, then it is harder to steal.

of course in real life pirates did not bury treasure very much. William Kidd is the only pirate that it is confirmed had buried treasure. probably some other pirates did it and nobody ever found out, but it was not really common. to bury a treasure would be a waste. if you have that much treasure then there will be some governor who will offer you a pardon in exchange for a cut. then you can retire and be rich for the rest of your life.

in the world of La Mouette Noire though, there is only one government, so it is harder for pirates to get pardons. a real pirate could steal from England and then get a pardon in Spain. but if there is no political reason, then a pirate will have a harder time getting a pardon. so that is one reason pirates in La Mouette Noire bury their treasure. another reason is that many pirates are NPCs, and while NPCs in La Mouette Noire have a certain amount of free will they are also programmed to do things. so if an NPC pirate gets some treasure, he will bury it because he is programmed to bury it.

Sarena, the mother of Mara, was a player, so she had complete free will to do whatever she wanted. in a lot of ways she was pretty smart, she did not try to carry all of her wealth around with her. if someone can look and see how powerful you are, such as how many lives you have or how many points you have, then they can plan on how to beat you. but if you pretend to have only 3 lives when you actually have 30 lives, the plans that your enemies make will fail and you can beat them better. that is what Sarena did.