Restricted free agents are valuable commodities for teams to have because they are cost controlled. Before the 2004-2005 lockout, players had few rights before they hit the age of 29 or 30, now those rights last through 25. This means that RFAs are a valuable commodity for teams to have and ones that they usually want to protect at all costs.
The reason RFAs are so valuable is they are cost controlled. Teams can hold them hostage and basically force them to sign a contract that is good for the team because it is well under market value. If a team does decide to trade a player, they can hold onto that player until the perfect deal comes along because the player player does not have the ability to just walk away at the end of their contract.
The NHL did think they had a way to give RFAs more rights with offersheets, but since the summer of Kevin Lowe there have been few offersheets. This is a shame because they are one of the best ways to force a rival into paying a player slightly more than their team budgeted for the player to only make a specific amount.
RFAs are valuable to their teams because they are cost controlled players. For some of their best years teams have the ability to sign them for well under market value. If a team plays it right, they can lock a player up for all their prime years for under market value. This is why teams love their RFAs and try to get the good ones signed for long-term as early as possible.