If you are on the 53 man roster, you are sole property of that team. Yes, you might not get activated for Game Day some weeks, but the team does not have to worry about losing you. If an injury happens, they can move a guy on the roster to the 45 man game day roster.
Having the 8 players on the PS gives the team a total of 61 players for practice during the week and again, if an injury happens, the team can move that player to the main roster from the PS.
The bad thing about the PS though is that if another team has a hole they want to fill, they can steal a player from your PS. For example, Anthony Armstrong is on the Dolphins PS. Lets say the Jets have an injury in practice tomorrow at WR and they like Armstrong, they can sign him to their 53 man roster and the Dolphins get nothing in return. It is a risk.