1: Electric locos:
The reason side rods were used on early electric locomotives is the size of electric motors in the beginning. Later when it was possible to build the motors smaller, they were placed between the driving wheels.
2: Diesel locos:
Side rods on smaller diesel locos are usually used, when transmission is either mechanical or hydraulic instead of using cogwheels/chains between drivers.
3: Steam locos:
On compound locos, system Vauclain, there are two sets of cylinders: First the steam goes to the upper high pressure cylinders, then being used twice, it goes to the lower low pressure cylinders with a common inside valve gear. This fuel saving system was mostly used in the US. In Europe the high pressure cylinders were usually placed between the frames, and the valve gear was placed outside the frames, utilizing easier maintenance. Generally these fuel saving, but rather complicated arrangements proved to be heavy on maintenance, and later most steam engines were built as simples with 2 or 3 cylinders, exept in France, where compound locomotives were very popular until the end of steam.