Possibly because there may have been mistakes in the forms they sent, they could have been lost in the mail, the Census Bureau could have never received them, or respondents could just be saying they mailed them in and for data collection purposes and accuracy, you just can't take their word for it. Maybe their household was incorrectly input and put under another address, etc, etc. As an enumerator, the way I look at it is: if you made the effort to already go to their houses and be there with them, a few minutes to complete the form shouldn't be too harmful to ensure accurate information retrieval.