It's absolutely critical for your growth and development to complete workflows that have been assigned to you on time. It provides an early sense of job satisfaction knowing you are catching on and ramping up to full productivity as quickly as possible. Its also important to show your team mates how on the ball you are and anxious to make an impact. Late completion makes the exact opposite impression on your team.
When you complete and then mark your tasks and check-ins complete, the workflow moves forward and other supporting team members are assigned activities (tasks or check-ins) to further support you. You are no longer a potential blocker for someone else downstream and in fact, if activities are not marked as complete then the workflow can stop at that point.
If you don't mark activities as done in a timely fashion then the platform doesn't know that the activity 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 activity). Help us not spam you by keeping the platform updated to everything you have done in support of your hire!
Lastly, check-in scores are critical to help your manager know of areas for development at the moment you really need their support. Delaying completion can actually affect the answers you provide and ultimately the support your manager provides when feedback is finally received. The longer opportunities for improvement remain unaddressed the larger the likelihood you will fall behind. Help your manager help you succeed by providing feedback on time and with color through commentary!