The Reds have been long-standing supporters of Great Ormond Street hospital.
Also, people do like to see the Red Arrows; add this to the package and it might convince more donors to go.
Those who attend will probably leave with a slightly warmer and fuzzier feeling about the RAF because the Red Arrows bothered to turn up.
And imagine the publicity if the team refused or were prevented from appearing because a bean-counter said 'no'.
Admittedly, there is personal and pecuniary gain - the latter for Great Ormond Street, the former for the sick children who just might, even if the link is indirectbe cured/enjoy a better quality of life thanks to the money raised.
Personally, I'd rather see the team support this sort of thing than be expected to turn up at taxpayers' expense to give Premiership football clubs which spend money like water the thrill of a cup final flypast...