Smart Motorway set to 60mph

Tonight we travelled (about 9pm ish) along the smart motorway south bound from jct 15 to beyond jct 13. The signs were all set to 60mph, there was literally no problems, no traffic congestion, no one in the hard shoulder, we all sat in the left hand 2 lanes, and trundled along at 60. Then after several blank signs, we made the decision that the 60mph must have ended, but there was no sign to show this.

I understand smart motorways, and stick to the rules. But if there is no reason for it, people will ignore the signs, and then when it matters people won't react as you need them too, and put people in danger. Please could yoy tell me what the purpose of this speed restriction was tonight, to better understand