Micro-USB Type B Cable
Although as of 2018 the most used USB cable for phones is the Type-C, the Micro Type-B is still used in some areas. They're typically used as a charger for phones or for other handheld devices, such as a GPS or gaming consoles like the Nintendo DS.
Function
Used with almost every mobile device aside from Apple and some devices made after 2017/2018, the Micro Type-B acts as a slave device rather than a master device. With a maximum power transfer of 60 watts at a time and a power current of 3 amps, these USB cables usually charge devices.
They can, however, support high-speed transfer rates of 480 Mbps with on-the-go features, allowing it to be plugged into host devices (i.e phones, tablets).
The Micro Type-B also holds the ability to data transfer. If one end is plugged into a phone and the other into a computer's USB port then you can transfer data to and from the PC and phone, meaning you can backup precious photos onto your computer or upload music to your phone for a new ringtone.
The 5-pin plug can be fitted into the Type-B and Type-AB female receptacle, as well as some two-in-one Type A and Micro Type-B versions, making it quite a versatile cable. This ability makes it useful in quite a few situations, hence the commonality in handheld devices.
Where it's used
You can find the Micro Type-B being used with quite a large majority of Android phones prior to 2017 and 2018. Some examples of its use are with the Samsung Galaxy S7 and S7 Edge, the Sony Xperia X, Motorola Moto G5 Pus and the Huawei Honor 6X. However, no Apple phones use the Micro Type-B USB on their device.