Although my cats are strictly indoor cats, I wash them once every 1-1.5 month. I have started bathing Minnosh when she was around 2 months old (now she is 3) so she got used to it. She is not scared of water; she visits me while I'm bathing, sits on the bath tub & watches me or tries to play with the water from the tap while I'm washing my hands
They clean themselves but they actually cover themselves with cat spit
And when combined with their natural oils, their fur become kind of sticky, or flatter or I don't know...When you wash them, their fur become shinier, fluffy & softer. Well, that's my idea
I think instead of baking soda, people use cornstarch baby powder for waterless bath. Never use talc powder however, cause it may cause serious respiratory problems when ingested from the nose/mouth.
Washing cats help some people who have cat allergies.