To do lists are for all of us to track our work and set deadlines for ourselves. When you add a to-do inside a ticket that you are working on, this to- do list is available on the dashboard as well. When this is complete, you could check this in the dashboard which would be reflected inside the ticket as well. 

A few things to understand about to- do list are that when you add to-do for someone else who is assigned to a ticket, this would be available in that ticket's details page, however, it would not be available on their dashboard.