Good question

To be honest, the important is that all members understand that unless someone that downloads keep on seeding the torrent it will die. And nobody wants that to happen right? Always irritating when you search for something, find it, but there are no seeders. Then we swear over others not seeding, but maybe we don't do our part ourselves in some cases? So we had to make some rules. We encourage all members to seed for as long as possible no matter if the torrent you download is new or old. While you have the space on your hdd and the computer is running you might just as well let the torrents seed.
The exact rule is to seed all new torrents as in all torrents that you download, for 48 hours, but please seed longer!
To seed torrents for a long time is a great way of gaining upload GB's and ratio. For example if someone comes along a year later and makes a search for something and you are the only seeder of a large pack you will get all the upload credit and so on.