My wife and I are on vacation in the UK and, while watching “Good Morning Britain” we would look at the time on the bottom of the screen and saw that it was one hour off. I Googled it and saw that it (hence the “+1”) was deliberate but I couldn’t find out why it was done in the first place.
So, what it is the purpose of this, beyond temporarily confusing tourists?
Something about timeshifting. Obviously the natives will know more, but I’m thinking you’re an hour behind the live feed.
What Joey P said:it’s just the same as the “base” channel, but with the programs shifted by one hour. Last time I lived in the UK there was another channel that shifted by 3 hours, I don’t know if it still exists. The purpose is simply to make the programs available to more viewers. We had a couple of attempts at similar channels in Spain (mainly on what back then was the only channel you had to pay for), but they’ve pretty much been displaced by the ability to view programs on demand through the internet.
Yeah, it’s a byproduct of the digital television age - there are a lot more digital channel slots available so many channels offer a +1 channel in addition. That way if you’re channelsurfing and you come across something interesting in progress, you can catch the beginning of it on the relevant +1 channel. Presumably this all makes economic sense too.
I don’t think there’s a +3 channel, but there might be a +24 one somewhere in the mix.
Fun fact: there is a UK television channel called “Dave”. Its +1 channel is called “Dave ja vu”. 
That makes sense, though it was a tad confusing at first. In Canada, on the morning CBC news, the time display cycles through our time zones with the relevant time zone labelled.
Giving rise to the classic line from, IIRC, Mock the Week: “Viewers are reminded that ITV2+1 is not the same as ITV3.”