The thing is, 0 can't be cast to a date.

DECLARE @SomeDate CHAR(8) = '0'

SELECT ISDATE(@SomeDate), CAST(@SomeDate AS DATE)

Msg 241, Level 16, State 1, Line 3

Conversion failed when converting date and/or time from character string.

So if you have thousands of rows with a 0 in, you will get conversion error.