Erm.....
1) This was the first in a series of promised daily news conferences. It wasn't called specifically in and of itself.
2) Johnson did not "do nothing". He announced that official Government advice - with immediate effect - is for everyone to avoid social gatherings of any sort, including bars, pubs, restaurants, theatres, cinemas etc. In effect, the Government is now telling UK inhabitants not to do any of these things.
3) The reason why the language presently being used is "you should not congregate" etc, rather than "you must not congregate" etc, is that at present the Government has no legal powers to enforce either the non-gathering of people or the closure of places such as restaurants, pubs, theatres and so on. And there are two factors driving this approach: firstly, the Govt hopes that all sensible people will abide by the requests, and that all sensible operators of relevant businesses (e.g. pubs, restaurants, theatres) will act on their own accord to close down their premises to all but take-away options. But secondly, if (or when) some people or businesses decide to ignore the requests, the Govt will at that point introduce enforcement legislation. By taking this two-step approach, the Govt avoids coming over as unnecessarily totalitarian; and the wider population will be much more amenable to the Govt using legislation and enforcement powers to deal with the small minority who refuse to respond to the current requests.
I've had a ton of emails this evening already from venues announcing they're now closing for the forseeable future in response to this evening's Government announcement. This is precisely what the Govt hopes - expects - all responsible businesses of this sort to do unilaterally, despite the deep commercial harm it will do to them. But people in the UK should be in no doubt: the Government is requesting that you stop all forms of social congregation - and if you're stupid enough and irresponsible enough to ignore the request, there'll be legislation and police enforcement to force you to comply before long.