@The_Abyss - You don't like guns and we'll never change your mind, I get that.
Instead, we're trying to answer your question, whether you agree with what we say or not is immaterial, we're just giving you legal reasons so you can have a better understanding of why other people enjoy/like/use firearms. Again, you don't have to agree with these legal reasons but you do have to understand what they are, two very different things.
Your problem is one of "definition" as the words semi-automatic ONLY refer to the way a firearm operates and nothing else. You can have semi automatic air-rifles, pistols, carbines, rifles, shotguns, cannons etc firing anything from a tiny 4.5mm ball bearing which can't puncture skin all the way to 203mm artillery shell and ALL of which are all "semi-automatics".
So you see why it's difficult to give you a list of a civilians uses for a "semi-automatic" firearm when you clump everything into a "why do people uses semi-automatics" but to try and answer, the biggest need (and the NZ Government agreed) is the use of .22 semi-automatics in Pest Control for agriculture. However whether you agree/disagree is moot, it's a lawful use.
In the spoiler below I've gone into why even saying ".22 for pest control" is a confusing nightmare just due to the definition of "what is a .22".
The thread's about semi automatic rifles. But for clarity, I'll save you the long definition and why I don't think that's important. I'll just replace semi automatic rifles with firearms. I don't have a problem with definition - firearms are designed to kill or wound.
Why do people need firearms. So far we have a use put forward of pest control. Unless there are a lot of pests in the UK or New Zealand, I've seen nothing to suggest that 99% or more people don't need to control pests with firearms. So they don't need firearms.
Any other need for firearms? I don't think that you can use 'legal reasons' by the way - that's part of the debate: making some things illegal that are currently legal. By definition, not all currently legal uses are valid. That's why this discussion has started.