Now Wimbledon is going on for over a week, I wonder why some players enter mutliple events on the same Grand Slam, e.g. the Williams sisters that in the past participated in the Women’s Singles and Doubles. There are plenty of players that start in the Single or (midex) Double.
First reason is Money as you stated.
Wining double on Wimbledon worth 260,000£ per pair meaning 130,000£ for each player.
Winning Mixed double worth 92,000£ per pair meaning 46,000£ for each player.
Second, a big tournament has it’s glory.
Winning a Grand Slam or an Olympic looks very well on your resume, No matter if it is not singles..
Last reason, It’s FUN, some tennis players love to play tennis! a.k. Andy Ram
also Williams sister love to play with each other and this is their chance.
It’s a way to meet and make friends
Double/ Mixed double require smaller effort then single.
As an example Roger Federer, one of the greatest, played doubles in only 4 grand slams (The last one was on 2003!).
But on 2008 Summer Olympics in Beijing Federer played and won the gold medal
So, to conclude: Money, Glory & Fun.