The risk of not meeting the planned dates, particularly the Late Finish (LF) date, can be mitigated by including some buffer time, as smk45 suggested. During the project planning phase, you want to make sure you account for and mitigate any possible that may arise throughout the project. However, as anyone who has worked on a project team before would know, unexpected risks can arise often in reality. The buffer time allows you to be able to handle unexpected developments without immediately risking getting off of the project schedule. The task slack and project slack that is created from the difference between LF and EF dates also accounts for the same thing.