We leave the EU 2 years after we submitted Article 50, so that is 29th March 2019.
If the date is put into legislation, it means that if negotiations are not complete by that date, then we will crash out of the EU with nothing to replace it. The ‘No Deal’ scenario.
If it is not etched in stone, then we leave ourselves wriggle room, where we can carry on seeking a deal after 29.3.19 if both parties agree. It is in both the UK and EU interest to have an orderly parting, and legislation that makes running out of time as the most likely ending, is not desirable, especially as 29.3.19 is a proper deadline anyway.