It's to prevent one single "faction" if you will from being in power when the majority of the country didn't vote them in.
Basically, the way that the British politcal system works is as follows:
Each constituancy has their candidates and each person votes for their local MP.
The PM with the most votes in that area, wins a seat in parliament.
If a "faction" has more than 51% of the votes then they get power over the country with their Prime Minister at the head.
If however, like what happened in the last elections, there isn't a 51% majority, then the larger powers are able to "team up" and with the say-so of the Queen (yes, she does something every once in a while) it is allowed if it above 51%. So, this time round we have the Conservative head while having the Liberal's being David Cameron's bitches (Nick Clegg has been done over and none of his promises are getting through since Camerons manifesto/manefesto goes against most of the stuff.)
(The Queen can also disolve the government and take full control herself, but that won't happen unless David goes mental and bombs the US or something. The Queen can call for a re-vote or something similar that basically means that the government has to be voted in again.)
Each PM stands for 5 years unless they want out before hand, like Tony Blair did, who ran off and tried to become EU Overlord.
Also, most of the south votes Conservative, most of the North votes Labour, Liberals are fairly "low key" and are deemed the next major party but still don't do as well as either of the other two. There are lots of smaller parties as well, such as the "Monster Raving Looney Party" and the "British National Party" (The one that basically wants to legalise pure Brits to run around the streets shooting black people & anyone not born in Britain and deporting them. Needless to say, the MRLP has more chance of being voted in than them!)
From what I understand, the reason behind this majority of majorities thing is to stop something like how einstein got into power whereby he won by a majority, although only by himself, not by constituancy votes. Otherwise there would be a bigger chance of Nick Griffin (BNP head guy) getting into number 10.
There is your brief overview of British Politics, I know no more but although it sounds pretty sad me being able to write that down, if you're at voting age in your country, i'm sure you can actually sit there and think about how yours works! ;)
