is there any reason to keep iOS installers in my MacAir hard drive

Thanks for that Jimmy CMPIT, most reassuring. I sort of thought they sounded like a heavyweight dmg file - a bit of dross after being used. I wonder at their using so much space (5.5 Gb for two apps) compared to a dmg file, which you'd imagine to require a deal more code. Thanks again. And also to you macjac, I'd concluded that if they should be needed again, they'd be fairly readily available - or more likely superseded. Good news folks, and quick off the blocks, too.