The simple answer is that, without a website that can be checked, there is no way of verifying or moderating the map entries.
Anybody can enter new data to the map. There is no need to register etc. So that makes it possible to have a genuine ‘crowd-sourced’ map that anybody can contribute to with the least effort and any entry can be updated by anybody*. But that means it’s easy to ‘spam’ the map with inappropriate entries, or just make a mistake in what is put there. Having a web site that anybody can check means it’s easy to detect errors or abuse.
Also I feel that, if you want to take advantage of the internet, which is a great medium for getting people to come to events, then you should be prepared to put a little back. Without some kind of web page, potential visitors have no straightforward way of finding out if the club is still running or what’s happening this week.
It’s important that the web page is about the club itself, not just a website for the pub or a directory list that may not be up to date. Sometimes a pub or other venue will have a page about current events showing clearly that the club is running so that would be OK, but having something under your own control is infinitely better. Here’s some advice on how to set up a web page with no expense and very little skill.
*NB. If a update is made by a member of the public, and the previous one was by a club organiser, I will pay special attention to checking that it’s appropriate, including checking with the organiser if possible.