Why should we make a SessionScoped ManagedBean thread safe in JSF?

I know that Application-Scope persists across multiple users, so it's obvious that we should make sure that all the ApplicationScoped ManagedBeans are thread safe.

I also understand that we don't need to care about thread safety for a RequestScoped ManagedBean. That's because it last only for one HTTP request, and is newly instantiated for each request if it's referenced.

But I am not quite sure why we should worry about thread safety for a SessionScoped ManangedBean. Even though it persists across multiple requests, each individual user gets his/her own instance of it, right?

So, why do we need to worry about thread safety in case of SessionScoped ManagedBeand, and does that apply to ViewScoped ManagedBean too? ViewScope persists across 2 consecutive requests for the same view, right?