Target Audiences

The reason we need to know our target audience before we make our shows is because knowing gives us the ability to make it more appealing to them. If you didn't know your TA then it could go completely wrong. For example, two different quiz shows, Pointless and Celebrity Juice. The producers know that Pointless, because it is broadcast at about 6 in the evening, is a family programme. So they have to choose their content appropriately, this is why they have no bad language, no sexual references, so it is suitable for any age to watch.
On the other hand, Celebrity Juice has to be put on after the water shed because the presenter can be unpredictable, there is bad language and a lot of sexual references. The target audience for this show would be 16-30 year olds, it would be very inappropriate for children.
Other reasons to know your target audience would be for advertising. During a programme like Top Gear, when repeated on Dave, as the show is more directed at men the products that will be advertised would be mens deodorant and other appropriate products.

BARB (Broadcasters' Audience Research Board) is a company owned by BBC, ITV, Channel 4, Channel 5, BSkyB, and the IPA (Institute of practitioners in Advertising). They monitor all TV channels to see how many views they get daily. They are able to see what sort of ratings each programme gets, this means, if a programme that is on later during the day doesn't get many views they can replace it with something else.
This is also the way that they determine whether a show is good enough to carry on after a pilot episode.
If a company really wanted to advertise a product on TV they would consult with BARB to see what time they could do it. Lets say a company wanted to sell female watches. If they had quite a big budget they would be able to put it on during prime time around one of the soaps like Coronation Street. This means the  product would go out to a lot more people but would cost more. If the company had a tight budget they could decide to advertise it during a show that has less views, this means that less people will see it, but they could choose a show that is more focused towards their target audience, such as Loose Women.