In the DateAdd function, third parameter is date. 2nd parameter is the increment.

When you write DATEADD(d,0,getdate())

0 days are added to current date.

When you write DATEADD(d, DATEDIFF(d, 0, @Today), 0);

the date here is 0, select cast(0 as datetime) -- 1900-01-01 00:00:00.000

I'm not sure what you are trying to achieve here.