If you want to use a default date in that column then yes, it works well but if you do not want to insert anything if there is no date then better insert a NULL into the column like this:

insert into tt99(datetime2, description)

values (NULL, 'Testing the Default')

Let's say for instance you have a field DateofBirth and it is not compulsory to enter anything then you would not want to enter today's date but rather a NULL.

Should this date be entered from a UI and the program returns an error when passing a NULL field then I would rather pass "" and in the SP put a case on the insert like this.

case when DateofBirth = "" then NULL else DateofBirth end

That's just my bit of advice.