Cars have rear view mirrors and so do motorbikes - both are legal requirements. So, why not bicycles or e-bikes? That's a darn good question and even though they are not legal requirements, there's nothing to stop you from investing in one. Especially when they are such affordable and practical safety devices.
Sure, you can always rubber-neck and just look behind you, but this does have a problem insofar that rotating your head can affect your balance and this in turn might make you swerve on the road. This is also made all that more difficult if you happen to be wearing a backpack. A mere glance into the mirror means you can see what's behind you without taking your eyes off the road.
Many cyclists, either consciously or subconsciously, rely on the sound of cars to judge whether there's one or several behind them. However, many modern cars can be very quiet and hybrids and electric cars are virtually silent and as such cannot be easily detected by the sound they make.
When yours truly went to driving school, we were taught to check the rear view mirror once every seven seconds so as to be constantly aware of our surroundings. Looking behind you on a bicycle with no mirror at anything like that kind of regularity is not at all practical. Also, if you are riding with a group of people or indeed with kids, it is an excellent idea to have a mirror to keep an eye on what's going on behind you.
While a bike mirror is a great idea and no doubt adds an extra element of safety, it is not a complete replacement for actually turning your head to take a look behind you. Even a brief bit of eye contact between a cyclist and another cyclist or a car can act as a moment of silent communication - it let's the motorist know you have seen them.
However, when appropriately used as a device to have extra awareness of your surroundings, bike mirrors go a long way for safety and simply make sense. Just make sure it's properly adjusted and clean!
By the way, our favourites are by Azur, ETook and Mirrycle - and you can see them right here.