It's absolutely critical for new hire's growth and development for them to complete workflows that have been assigned to them. It provides an early sense of job satisfaction and a ramp to longevity at the company. Its also important for their support teams to further reenforce the global values that stem from timely execution of activities for which people are responsible for. In work teams, often others are inconvenienced when people miss deadlines and deliverability suffers.
When you complete and then mark your tasks and check-ins complete, the workflow moves forward and the new hire and other supporting team members are assigned further activities (tasks or check-ins). 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.
As above, timely completion of activities also shows your new teammate how much you and the company care about them and their success. Its a stressful time for them. If they feel like they are falling behind because of lack of team support then they might become disengaged and disillusioned over time. That's a fast ramp to dissatisfaction and attrition.
If you don't mark activities as done 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 the evaluation of a new hire's performance and development at the moment they are asked. Delaying completion can actually affect the answers you provide and ultimately the support the platform recommends when feedback is finally received. The longer opportunities for improvement remain unaddressed the larger the likelihood your team mate will fall behind.