It's absolutely critical for your growth and development to complete workflows that have been assigned to you. It provides an early sense of job satisfaction and signals to your manager and team mates how on the ball you are.
When you complete and then mark your tasks as complete, the workflow moves forward and other supporting team members are assigned further activities (tasks or check-ins). If tasks are not marked as complete then your workflow can stop at that point.
Lastly if you don't mark tasks as done then the platform doesn't know that the task has been completed. In that case, the system will continue to send out reminder messages. This could clearly become annoying (especially if you actually completed the tasks). Help us not spam you by keeping the platform updated to everything you have done in support of your hire!