Well, as we all know, countries have their own boundaries .. you know why?
it's just like houses which has their own boundaries to protect their property..

so about your question .. scientists divided the lithosphere into several plates to create boundaries... 'lithosphere" is important to humans and all other living things... lithosphere is just like our father.. they are the ones who supllies almost all our needs.. that's why scientists divided it it to preserve the earth...