Often these designs will be created during flash, and their effectiveness is something that is debated among web design professionals.
The plus side of such an introduction page is that it creates a multimedia experience from the get go, giving you a display you can show off to your visitors and to set the tone for the rest of the site. These introductions are often very slick and they can make the site look very professional and help to make it more of an `experience` rather than a place they just read things. It says `welcome to the site` and it says `look at how much thought we put into our design`.
The bad side is that by its very nature it takes longer and that requires a certain commitment on the part of the visitors who will need to sit through the design or navigate around the logo to find the entry link. This also suggests generally that the site will be designed more this way and that they can expect to take longer in the next stages too.