For the same reason we have terms for stages of human development. We don’t just have the words baby and adult. If we want to talk about the stages of development of anything then we have to provide definitions for those stages. They may not be relevant for the vast majority but if you were going through these stages and you were working with someone that had already been through them - you would really want to be able to have some way of communicating about and understanding what is going on.
I don’t get the sense that these stages are at all subtle. Look at the descriptions from Ajahn Boowa for example or Thanissaro’s ‘awareness outside of time and space’ as a description of stream entry.