Coming up M6 today heading for scotland and all the overhead signs said “traffic diversion for Scotland use M6” it had me for a second I thought I’d got lost somehow.
Probably for those drivers who have to plan a long distance route. Don’t see a problem with that really. You wouldn’t either if you used the M1 and got stuck in traffic… That is assuming the M1 went to Scotland… Isn’t that up north somewhere… 
No, I do know where Scotland is, really. I have been to Inverness previously but, well, I had a map with me then 
I saw that yesterday on the M6 on my way to Glasgow, I was assuming there were hold ups on the A1 ?
weather was not nice with high winds and sleet.
I did have to smile at some keen “spotters” at Elvanfoot in the sleet/high winds mid morning but they were still there mid afternoon on the way back ! That must be some serious hobby!
Going up the M1 yesterday morning the signs from were I joined a J8 were showing M1 Closed J29 - J29a and from about J15 some signs said For Scotland Use M6. Once onto the M6 the messages on the signs were the same as you saw.
Scotland isn’t just Glasgow. If you were going to Edinburgh, mileage wise you would stay on the M1 until Sheffield.