There is a tendency with clients to add a lot of things to their websites or advertisements. The logic is simple: If there's space - why not use it? Get something in there - anything. Use that space - it will help us to sell more.
This kind of approach leads to overcrowded and cramped designs. Another downside to this is user distraction. The extra item, logo or link that has been added now distracts the user from the original goal of the page or advertisement. The real point is to funnel the user towards a particular destination or action. If they get distracted - they leave and the "conversion" of interest into a sales lead is lost.
We encourage simplicity, its a better approach. Especially on the web, where users have a reduced attention span. Theres a statistic that says websurfers have a 70% shorter attention span than magazine readers. So a simple, clean look using lots of whitespace is the way to go. Oh yeah, one more thing, the white space doesn't always have to be white. It can be any color!