Similarly if you don't know what to ask Wikipedia for you are never going to find it. For example if there is a dustman's (or as they are know doubt called now a peripatetic disposable environmental logistical transition officer) strike and you know nothing of the Winter of Discontent than you won't be looking for it and more importantly you won't feature it in your story.
Some years ago I spoke to a reader while on a local newspaper and he complained: "The problem is that most local reporters are just thick. The editors are usually alright - but even then sometimes..."
It is not my view. Although given the paper I was working on at the time I could see how he may have reached that conclusion.
As a general rule of thumb I'd say it is worth having a detailed understanding of the recent (five years) events), reasonable background on the past 100 years and a decent amount of knowledge on overall British history.
You see, unlike science - people will expect you to know something about the past. (Scientists are a far more understanding mob than most people and they know the chances are that the day you flunked your GCSE chemistry was the last day you ever looked under a microscope. A little tip when interviewing anyone esp scientists about their work wait for them to finish their explanation before picking up your pen and then asking if they could put it into layman's terms for the readers...you understand the concept, of course, but you feel their words would be so much better. (This usually works until the day that someone responds that they WERE using layman's terms and your general ignorance is revealed)).
So why is history important? The chances are if you are working on locals you will have to cover the usual fare of anniversaries these are usually dull and uninspiring reads generally brought about by a complete lack of interest on the behalf of the reporter to the subject matter. By knowing of the events we can better understand the person's role in them and contextualise it within our story.
Again this is not about having a detailed history - we are still writing a story - but a general over view that stops us looking ignorant. It is a surefire way to lose the respect of the interviewee..alternatively a little knowledge can go a long way.
For a readable overview of British history it's worth reading This Sceptred Isle which romps through the 20th Century...there are also CDs and, no doubt, downloads.