#2. To save the main event of night 2

WrestleMania is too big for one night, as WWE has been advertising this year. The truth is that the decision to host it over two nights in two different venues was due to the social distancing protocol and the logistics of having two separate crews.
The original main event of WrestleMania36 was set to be Brock Lesnar vs Drew McIntyre, but a two-day WrestleMania meant that WWE could have the WWE title match headline day one while the Universal title match headlined day 2.
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When Roman Reigns pulled out of WrestleMania with only days to spare, it became a big problem for WWE, which is why they were lucky that they didn't book Strowman in any major match.
It would be hard to imagine any other match headlining night 2, with only the RAW Women's Championship match having an argument to take that spot (should the Universal title match have been canned altogether).
Either way, the decision to put Strowman in was clearly to save the second night of WrestleMania.