Saw this while browsing PlentyofFish:
Please enter your income, we use it for matchmaking and it will never appear on your profile! You will not show up in the rows of images on top of pages, My Matches and many other sections of the site until you do.
This is also probably used for courting advertisers and targeting purposes. Nothing to keep people from selecting whatever they want, but a very direct way to force people to provide additional information about themselves.
Getting people to fill out their entire dating profile over time without bogging them down at the time of signup is something I’ve helped a lot of dating sites with. As with all new features, Always Be Testing.