Pressing L+R also help the switch know in what position you are holding the joycons.
If you press L+R the Switch knows you are using both joycons at the same time (and knows in which mode you are playing thanks to the bluetooth or the little connector of the joycons [Joycon detected and it's being used: It's via bluetooth? Yes? either TV or Tabletop mode / No? It's portable mode or joycons are attached]).
And if you press SL+SR instead, the Swich knows you are playing with one joycon sideways, and treats every joycon as a separate player.
You can test this out by going to the sync joy cons page on the home menu, pressing "Change Joycons order" (may be called differently, in Spanish it's called this way

) and pressing eiter L+R or SL+SR.
That's why Nintendo decided to put "Press L+R/SL+SR to begin" instead of "Press A/+ to begin"