The Mariners traded their two top prospects to the Reds for pitcher Luis Castillo:

The Reds traded Castillo to the Seattle Mariners on Friday night for four minor leaguers, including the Mariners’ top two prospects, according to Baseball America’s rankings. Castillo isn’t a free agent until the end of the 2023 season, but the Reds hoped to maximize the return of a Castillo trade by dealing him with a year-and-a-half of team control left. 

MSN.com

That includes two shortstops and two right-handed pitchers. It’s a good idea to stock up on shortstop prospects. If they hit, they can often move to other skill positions if there is a log jam at short, or they become trade bait to fill in holes.

USS Mariner analyzes the trade from the Seattle side of things:

While Fangraphs likes the M’s offense due to their home park, they don’t score a lot of runs. Again, the park is part of that, but so is their nature as something of a low-average, boom or bust operation especially after Ty France. So why upgrade the rotation?

Three reasons. First, it lets them stick around in games when the offense isn’t scoring. Think of yesterday’s game, when Logan Gilbert kept the M’s in it, leaving with a tie game. They’re going to need a lot of such games to go their way, and as good as the staff’s been since June, they just acquired a pitcher with a better track record than Robbie Ray.

Second, pitchers like Gilbert and George Kirby are getting very close to innings limits.

USSMariner.com

In an earlier discussion of a potential trade for Juan Soto, USS Mariner noted that a big trade has to hurt, and the trade for Castillo fulfills that criteria as well. Given the prospects moved, a trade for Soto would appear to be off the table.