I think the biggest advantage is that if there is a crime just off reservation the tribal police can often get there quickly. Think of how long it would take if a crime took place just outside the Neah Bay boundary. The tribal police could get to the location quickly and secure it, then hand it over to the sheriff. In return, if something big happens on reservation or someone tries to flee, the sheriff can assist. Even with the agreement, both sides simply hand over the case to the other side. I know in Pierce County the Sheriff detain most drunk drivers until the State Troopers show up and then take over. In the same way, they try to hand off other criminal cases to the Sheriff. I was involved in a case where we stopped a fall down type drunk driver. The sheriff responded, detained the guy, then waited another 10 minutes for the state patrol to show up and make the actual arrest.