With this move, it likely closes the door on a potential Rob Gronkowski trade despite not knowing for sure whether or not he will play in 2018. It also signals that the Patriots were not sure if they could re-sign Cooks after next season given the money floating around the wide receiver market.
Cooks was very solid for New England in his lone season there. He caught 65 passes for 1082 yards and seven touchdowns and was one of the Patriots' best down-field threats. However, Cooks was knocked out of Super Bowl LII in the first half when he sustained a concussion trying to unnecessarily hurdle over Eagles safety Malcolm Jenkins.
That the Patriots were able to get a first-round pick back for Cooks after only giving up a fourth to get him from the New Orleans Saints last offseason is a big win for Bill Belichick. With the Patriots deep at wide receiver, Cooks was expendable.
For the Rams, it gives them the vertical threat they lost in Sammy Watkins, who skipped town for the Kansas City Chiefs. The move also makes them a clear threat in the NFC after acquiring Aqib Talib and Ndamukong Suh among others.