people love the joke about the legal tampering period, and though it is probably different than what NFL intended, I think there is value to having a few days for publicly known agreements where players could see if they misjudged with their agents and still had opportunity to back out.
It isn't common, but if so, it allows a player who may be in their "one shot" at a big deal to evaluate. Their agent will get crunched, but it's their fault anyway.