#2. The person who eliminates him will most likely be his WrestleMania opponent

This is the basic and most obvious reason as to why Brock Lesnar is in the Royal Rumble 2020 match. For the last two years, Lesnar has defended the Universal Championship at the Royal Rumble and as mentioned earlier - this is the first time in 30 years that the WWE title isn't on the line at the Royal Rumble PPV.
This is mere speculation, of course, but based on the rumor mill right now, it appears as though the Royal Rumble winner will be a SmackDown Superstar. The current name which is being passed around is Roman Reigns, who would face "The Fiend" Bray Wyatt if that happened.
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That means that Lesnar is going to need another contender and the person who eliminates him instantly has a claim to a WWE title shot. It's the easiest way possible to set up a WrestleMania 36 match for Lesnar.
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