I find that there is something very satisfying about manually bolting, especially when you are doing it as fast as you can to take down several targets. If I had never played ro1/2 I probably would have thought that it would be a silly idea that simply pressing the mouse button again could increase immersion on a weapon so much, but it does.
I think it adds a psychological element in terms of how a player interprets the slower firing speeds compared to a semi auto as well. On most games that have both bolts and semis if you are stuck with the bolt you spend a lot of time thinking "argh so much downtime between shots." I never really feel that on the ro1 games, because you are actually - in your mind at least - doing something.
Also, once you are used to it, it becomes almost sub-conscious.