When running a Docker swarm there are some container/services that are realy handy to have. You want a graphical representation of your Docker swarm? There is a service for that. You want a UI for managing your Docker swarm? There is a service for that. You need to distribute your own build images to your Docker swarm? Yes there is also a service for that.
Portainer is a Docker host and Docker swarm management UI, with this service you can manage your Docker swarm, create new Docker stacks and Docker services. You can see logging of the Docker services, and even login to de console of the container.
Visualizer is a service which shows you the current running services on your Docker swarm.