Established superstars don't need to worry about using spring training to fight for a roster spot. Prospects can make a strong case but depending on roster availability, making the team may not be totally up to them. Of those that don't already have a guaranteed place on the team, the ones who have the best chance of getting called up are arguably relievers.
Seattle stocked up on plenty of bullpen arms this offseason, giving themselves more than enough optionality in the pitching department. Most of these are waiver claims and minor league deals that won't amount to much, but there's one arm that has been utilizing this opportunity to the fullest.