If all the Immigration offices had the same rules and all the Immigration Officers obeyed the rules and interpreted them the same way, you would be correct.
Unfortunately as we all know this is not the case, even in the same office different officers say different things.
How many people on this forum have posted about problems with immigration, I got to admit sometimes its the fault of the applicant, but other times it seems the IO deliberately makes things difficult.
If you live fairly close to the Immigration Office and if its just a missing form or something, usually its not a problem, but some folks live hours away.
It seems Immigration change things many times without informing anyone, so you turn up and hey, you need to fill this in, or copy something.
Good agents take away the anxiety and can give folks peace of mind. You have to pay for the peace of mind though, but some think its worth it.