What is the importance of IA and UX?

This question has just popped up in my studies, retraining part time to become a software developer. For some reason this one has kinda flummoxed me and I can't really find the answer I want through research other than, software needs to be laid out logically, else people won't use it or can't use it. Any pointers to head me off in the right direction? got to cover two reasons in detail and the detail is what i'm lacking!