The Colts are looking to add as much depth as they can on their team. They want competition at every position on the roster and want to have the strongest roster. That is the only reason I can think of for working out the guys that they worked out today.
Wide receiver Brandon Marshall, running back Mike Gillislee, and running back Akeem Hunt all were working out for the Colts. Marshall is the biggest name of them all. Marshall last made the Pro Bowl in 2015 and played in seven games with the Seahawks last season.