Splash pages used to be used in abundance but companies looking to get their website noticed. Today, they are still used by some business and there are a number of reasons in which a company may want to do this, often it is to improve their presence on the search engines such as Google, but it may be to create a page that they can promote through offline marketing or to help with paid for online advertising.
If you create a PPC (Pay per Click) advertisement, then you need to link it through to a page on your website that is highly relevant to the advert. This will not only help improve the quality score for the keyword (keeping costs down and showing it higher in the listings) but will often help improve conversions. When people click a link on Google regardless of if it is a paid for advert or organic listing, they want to be taken to the page that is most relevant to the search term they entered. If you can do this then you will often find that Google and other search engines such as Bing will reward your site by pushing it further up towards the top of the search results.
Try and make each splash page unique and highly relevant. Also, make it easy for the viewer to navigate to other parts of the site from the splash page.