Working with ESLInt in Visual Studio code you might run into this error

elements in iteration expect to have 'v-bind:key' directives.eslintvue/require-v-for-key
[vue/require-v-for-key]
Elements in iteration expect to have 'v-bind:key' directives

Checking the Vue documentation at https://vuejs.org/v2/guide/list.html#v-for-with-a-Component you will see this

in 2.2.0+, when using v-for with a component, a key is now required.

At Stack Overflow thread https://stackoverflow.com/q/47395535/460885 you get a better explanation based on the (older) documentation:

To give Vue a hint so that it can track each node’s identity, 
and thus reuse and reorder existing elements, you need to provide a unique [...] 
It is recommended to provide a key with v-for whenever possible, unless the iterated DOM content is simple, or you are intentionally relying on the default behavior for performance gains.

So adding the v-bind:key does not hurt either way.