They're not necessary but they do keep the dirty diaper smell down. But that same thing can be accomplished by taking the dirty diapers to the outside trash can as soon as possible.
They're worth it though and do work for keeping the diaper smell down. But it's an extra expense that you most likely won't use much.
Unless you have several kids. And then you have the added cost of the plastic bags for refilling the diaper genie.
People got by without them for years so you can too if you don't want to buy a diaper genie.