Roster management is mostly an educated guess. Nobody can be certain about a player's future production, but MLB scouts and executives believe in their evaluations.
Every year, those assessments lead to offseason trades. Hopeful contenders pad their lineup and rotation for a championship run, and rebuilding teams part with key contributors in exchange for prospects and a brighter long-term outlook.
Not every decision pays off, though.
Big-name players struggle in a new location, or top prospects don't develop as expected. The buying team trades a future star, or the selling franchise gives away incredible value for little in return.