It sounds like I do very similar to Myusername (designing forms, training staff what to record, where to record it and why we need to, finding gaps, repeat ad nauseum) but should point out the NHS doesn't sell data to make money, just in case anyone got that impression. They get more money for having higher rates of completed data, which is paid by CQUIN to each Trust.
But occupation is also clinically relevant and others have explained why. Religion is clinically relevant, but is also essential to know if you're on a ward - for meal planning, helping patients to observe prayer times and of course, some religions have strict time frames which must be observed in the case of death, so hospitals need to be aware if they may need to fast track a post mortem in order to release a body within a respectful amount of time.
If this info is on your GP summary care record it's easier for the hospital to pull all this info across to their patient admin system if you're admitted in an emergency.
Your GP is doing good to ask these details each time, it can be a bit of a battle getting hospital staff to ask these questions, as unless it's explained explicitly why it's needed they think the same way you did at first OP and think it's unnecessary, or worse, being nosey.