It's the beauty of the messaging...we're at stay alert but use common sense.
Its the perfect message for an inadequate response, because everybody thinks they're blessed with an abundance of common sense and it's others who are the problem.
Anyone who says it's meaningless can immediately be dismissed as lacking common sense, a condescending elitist or playing politics.
It shifts blame for any negative outcomes onto whichever sub-group the media, social media decide haven't exercised any common sense - it'll be scroungers and forens.
And it means that the Government can say 'well we gave you the chance to do the right thing but you didnt' so people will look around and think 'well someone didn't do the right thing...' and take out their ire on them.
The government shifts from one that offered liberty and freedom and left the common sense choices to the public to one that trusted too much in a public that wasn't responsible enough. Negative impacts now mean the Government tried to provide autonomy but members of the public didn't have enough common sense to do the right thing (see Grenfell).
It allows the Government to not have to deal with complex and difficult choices.
Detailed answers require difficult moral, economic and ethical decisions and 'best worst scenario' judgement.
"Just use common sense," pushes those decision onto others. The Government takes the credit if it works as they trusted the public to do the right thing, but if it doesn't work they tried to let us have our autonomy but WE made bad decisions.