VDom and Key attributes

I came across this discussion in the router component docs.

“To give VDOM a hint so that it can track each node’s identity, and thus reuse and reorder existing elements, you need to provide a unique key attribute for each item.”

I haven’t been doing this in my list views and it seems to be working well. My question, is the key attribute a best practice here? If you don’t include the key attribute, what is the downside effect?

One of the reasons I didn’t implement the attribute is because it’s not in any of the list view examples. If this attribute is important, perhaps it should.

If in your component templates you use {{each}} to render list items (makes them dynamically) and later dynamically add or remove items from list then yes, key is mandatory