It’s actually not too bad mate. No you don’t need to grab the ISO if you already have the 7.0 disc / ISO and the 7.5 updater.
The ISO is handy to have if you ever re-install from scratch again since it will let you install the full 7.5 without installing 7.0 and then having to apply an update. Most other software of a large size works the same way to be honest.
It’s good that Steinberg provide fresh ISOs of their 0.5 releases, which is why I’m trying to get my hands on the 6.5 ISO too since I currently have to install 6.0 then apply the 6.5 update. Certainly no big deal, but a nice to have 