In addition to consuming energy, it's normally safe to leave a USB cable plugged in. This is not likely to cause damage to the equipment. USB sockets are also extremely safe to use unless they are of poor quality. A cheap outlet, like any electrical outlet, can overheat; at the very least, it can take longer to charge the devices.
In a nutshell, unplug cables when not in use. Not only can this prevent this type of unexpected power consumption, but it also reduces the chances of ports or connectors being accidentally damaged. On a computer or laptop there is a downstream charging port that can supply up to 1.5 A, while allowing a USB to be detected and started charging before enumeration (which is when digital communication occurs). For example, some wall chargers can supply more energy than others and a particular USB socket on a laptop may vary in terms of power from the others, or computers, some of them, can be charged while in sleep mode.
This is another third-party cable, this one has a bright LED in the connector and, although the power consumption is still very low, that bright LED more than doubles the amount of energy the cable consumes. Power banks are generally rechargeable lithium-ion or lithium-polymer batteries that are used to provide a portable power source for electronic devices.
