The planning that is in place was put in place when the fear was that something devastating like the H5N1 (avian flu) virus seemed like a threat (and TBH, if it mutates to human-to-human transmission, it still could be, but hasn't yet).
In that scenario - where in the Far East 50% of all sufferers died and almost everyone who came into contact with it in chickens actually contracted it - such a human to human spread where the severity stayed high would be utterly devastating.
A combination of illness in adults and their children keeping people away from work, and people self-quarantining due to fear, could lead to large proportions of the population absenting themselves from work, including from essential services and supply chains.
In that scenario, there could be real disruption to daily life and real serious consequences - eg if 40-75% of the population were ill and large numbers of them had complications and large numbers of NHS staff were also off ill or as carers then you have a potential nightmare.
The govts and WHO are being cautious and trying to slow or limit the spread of the disease because with a novel virus which no-one has immunity to and which can mutate quickly to become less or more severe, no-one knows how serious or how little an impact it might have.
This is the nature of planning. The plans are being implemented not because this is 'normal' flu but because it is a 'new' flu.
I have to say the ONLY conspiracy theory I am prepared to entertain though is that this might be a global media/public information exercise - ie in preparation for the real nasty pandemic we might face, the WHO are testing the public and media response.
But - if you listen to what doctors and scientists are saying - there is real cause for sensible concern: it is not nothing.
Typically, it is hard enough to get academics (and medical scientists in particular) to say anything at all - they are typically circumspect: when they say 'it could be serious' they means it.
But the word 'could' is still in there. Yup, it could not be serious. But the plan is in place in case it is.