Ah, yes Sam that's a good one too. When I have friends, or my wife, at a gig I often ask "how does it sound"? Most of the time they say its great, but sometimes I get the occasional you need to turn something up or down comment. Now I can just hand them my phone and ask them to do it for me.
My wife in the past has just given me the signal from the crowd about something being wrong. Maybe she'll point to my guitar and give me the up signal, or the same with a vocal mic etc. Now she doesn't have to give me any signals. She can just pull out her phone and fix things for me.
Of course this idea needs to be reserved for people we know and trust.
I can't tell you how many gigs and concerts I have plays where I am my own sound guy. It's been thousands. I often make the statement how it sucks being the sound guy behind the mic. It's not always possible to know what's going on out front. But I must say that the iPad controlled systems are helping a ton. Being able to walk around a room/venue in front of the speakers, and away from the board, to adjust sound really makes a big difference. I get to play my guitar (wireless) and jam with the band to check the mix. I sometimes use a wireless mic and have brought that out too. But still when back behind the foh system a little tweaking might be necessary.