I didn't realise how much cats depended on smell to identify us and each others, until I bathed one of my cats with a perfumed soap - once I dried her and let her free to mingle with the other cats, they suddenly started treating her as a stranger and went into defensive mode, growling at her. Even she was confused for moment to what was happening and why her siblings were suddenly acting like that. I calmed them down somewhat by rubbing a towel over their bodies and rubbing the same on her sister's body. They finally settled down when they all had sniffed each others butt and perhaps also recognized her through her meows to me.
We use scent as one way to enrich our cat's life. Whenever we come back from, say, a walk, we let the cat sniff our hands (particularly interesting when we meet other cats on the walk!) When we are putting away a food shop he posts up on his stool and gets to sniff everything before it goes away. He is only really interested in protein smells and other critter smells, but the whole business must be engaging enough that he keeps coming back to sniff the tomatoes etc. as well.