In other words, it is the same reason that democracy is superior to communism or monarchy. When a group of people (communism) or a single person (monarchy) get too much power they become corrupt. And denominations, sadly, are no different, in church history the Borgia popes are a good example.
You may have read or heard about the novel Animal Farm. It uses animals to parody communism, but the end message is that absolute power corrupts. We have seen this so many times over the the history of the world. That is why it is preferable to have a democratic government, it spreads power because people can choose the best leaders and thus it stops empire building.
God knows all this, and so having many denominations actually minimises problems in churches.
There are other benefits in having multiple denominations. Locally we have both the CFS (Country Fire Service) and MFS (Metropolitan Fire Service). They both have the same aim - to prevent and put out fires. However they use different methods and target different areas, so having two organisations makes sense.
In the same way, having different churches of different denominations in an area should mean that more people can be ministered to.
There are differences between churches, some are important and some not so important. E.g. styles of music is one thing that is ultimately of little importance, but suits different people's preferences.
And in practice, there is actually a lot of activities where people from all sorts of denominations get together to support each other and work together, so it is possible to over emphasise the differences.
However, often there are important differences. For instance, some churches seem to have forgotten why they exist. A bit like a CFS group forgetting they are there to fight fires and instead thinking that they exist just to maintain their equipment.
So how can you tell which is church is right? The answer is actually quite simple, see what the Bible says for yourself - read it! It's not complex, and if you approach it honestly seeking to find what God says in it, you will find the truth. I suggest you start at one of the gospels - Matthew, Mark, Luke or John.
If you do this you will find that the overall message of the Bible is:
Some denominations that have lost their way add to this, some take things away. Those that add to the message often say we have to DO something, go to church, get baptized, go to mass, anything other than simply relying on what Jesus has DONE.
Others that take away from this message may say that they don't believe man is sinful, or that those who reject God won't really go to hell. This may express itself in being solely concerned for the poor, or denying that things God says are sinful really matter.
But you do need to make a decision. Not making a decision IS making a decision - it's making a decision to reject Jesus and his offer of reconciliation with the creator, and that has eternal consequences.