No network remains the same size. At some time, all networks grow. If you take future growth into account, there will be less disruption to your existing network when that growth happens. For example allocating a block of addresses for servers can easily prevent having to re-assign IP addresses to multiple servers due to a new server.

With static routing, if you group parts of your network properly you can easily use network masks to address each part of your network separately. This will reduce the amount of administration required both to maintain the routing, and to troubleshoot any problems.