#4 It opens up new feuds for him

One huge positive of moving Reigns to SmackDown is that it opens the door as to the new feuds for both him and the fans.
While he was on Raw, Reigns routinely feuded with Braun Strowman, Rollins, Brock Lesnar, the Miz and even Styles right after Reigns won the WWE Championship from Triple H a few years ago at WrestleMania.
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Even recently he had been feuding alongside Rollins with Drew McIntyre, Baron Corbin and Bobby Lashley.
And if Samoa Joe is moved to Raw because he is the US Champ and the Intercontinental Champ, Finn Balor, is now on SmackDown, then that is another feud he won't have to rehash.
Now that he's blue instead of red, Reigns can feud with Kofi Kingston (potentially), Lars Sullivan, Shinsuke Nakamura, a heel Daniel Bryan and even Randy Orton if deemed necessary.
He has already toppled the likes of Sheamus and Rusev, but the fact is there are more storylines to be had with a bevy of new opponents.