Read about this on a TheHoth blog post and it makes a lot of sense to me. To summarise:

Targeting your Home page to a whole set of keywords is not a good idea. If you have other pages which are also targeting some of the same keywords, then Google can get confused about which page it should be ranking higher, your keyword specific page or your Home page. 

So the theory is that each major keyword should have its own page that is optimised for that keyword obviously and that the Home page is reserved for branding information so that it is found by people who are specifically looking for you by brand name/ business name.

I think I have made that mistake in the past by targeting both my Home page and another keyword specific page for the keyword 'wedding magician'.