mairomaster , 04-14-2016, 02:40 AM
Robert can answer to this better, but I can give it a try.
The impedance control single ended tracks are normally required by particular interfaces - DDR memory for example. Common required values are between 45 and 55 ohms or so. When you use such tracks for several interfaces on particular layer, it is convenient to use the same tracks for all remaining signals, just for consistency.
Slow speed interfaces like UART, SPI or I2C normally don't require impedance controlled tracks. However it doesn't hurt to use the same tracks for them as well.