Because it is much easier than performing complicated modifications while the site is running. For example, at Google "down for maintenance" is not on the tabl

Because it is much easier than performing complicated modifications while the site is running. For example, at Google "down for maintenance" is not on the table. That can in some cases lead to lots of extra work or time, e.g. dual writes for a period of time followed by mapreduces to fix the remaining part. My internet bank is often down for maintenance on Sunday nights. I assume it is because they have a very old system.