The main reason to use an external pulldown (or external pullup) instead of the internal pullup would be to match some requirements for the external device that you are connecting. A pushbutton switch, for example, is ether fully open or fully closed, and can have its other side connected to either ground or +V, so there is little reason to use an external resistor. Some sensors or other external devices will have more limited choices. A typical example would be I2C (twi) devices; the I2C protocol specifies an external pullup in a typical range of 1k to 10k ohms, so the internal pullup is not ideal. TTL logic devices are famous for having a "standard" pullup value of 2.2k, because their inputs actually consume some current.
A requirement for a pulldown is actually rather uncommon, except that it allows switches to match "expectations" that HIGH means "On"...