Why are these customers doing server housing with us, even though they are connect to the Internet via leased lines themselves in many cases?

The answer is: speed.

All servers here are 2 milliseconds away from all other German ISP backbone nodes. Take any ISP you want to. Check out our looking glass to see how fast packets travel from here.

The additional roundtrip time which would be incurred by any leased-line to the customer by anything less than a 2Mbps line would be a whopping additional 30-40 milliseconds. This latency is actually quite noticeable. And of course the throughput is much less. Our richest customers can afford themselves a E1 line (with 2Mbps). This pales against the connectivity from here.

Housing with us (or renting space on our servers) is better, faster, and also much more economical.