It does if a person is introduced to Nostr by the platform, not the other way around.
So a user would start by using accounts. Multiple platforms with different accounts. Then they learn Nostr can do something that they like (like a Nostr standardised backup) or reach someone that they didn't realise they could.
Just to enable that experience, the platform needs to sign and publish on your behalf.
That is why. Because not everyone will join Nostr first, but if a platform they use uses Nostr and the feature it provides is beneficial, the user might want to explore Nostr but we should reduce the friction of having to start over.
