Film grain: Basic question

[KEROUAC1957]

I realise this might be blasphemous to ask but I have a basic query about film grain which I'm asking in all innocence. When I read bluray reviews they often talk about the natural film grain being present (or sometimes not). I'm not that knowledgeable about the technical aspects of film or bluray but I often wonder why people talk about the presence of film grain being important. I understand that film shot on film will have a natural grain and over use of digital processing will smooth this over. How does film grain improve your enjoyment of a film and what are the reasons for needing it.