Probably they were concerned that users have to store the secrets (certificates, etc...) in the container filesystem - or mount/share the location on the host were the certificate for a specific app is present. Probably there are more ways to do it, but also probably none is end-to-end secure.

With this you can securely store the certificates; takes away the developer's responsibility of taking care of managing and distributing them to their apps, swarm will take care of that in a secure end-to-end fashion.