It's required by "RV" code. I always take them completely out. Most of the one's I had, leaked at some point. By their code, it has to be mounted at / above the toilet level. That's why you see them under the sink a lot, or sometimes in a wall. It is supposed to prevent black tank water from going back into your hose at the inlet. But, if you look at the way the systems are installed, and how you flush a tank, this is unlikely to ever happen. We use a designated hose regardless. Assuming you've removed it like I have, to contaminate a hose, you'd have to have the tank above half full submersing the nozzle in the tank. Then it'd have to syphon out of the tank through those tiny jet holes.
Regardless, you can replace it if you want. The next one will probably leak at some point, and next time you may not be watching it and create some water damage.
Ever look at an aftermarket set up? Some come with a vacuum breaker, that installs right at the inlet. There is no "above the toilet" breaker installed.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.