The reason to state marital status is to learn whether there is a possibility you might have half-siblings. Therefore potential immigrants.
For example - my father married two times after my mother. As a result I have two half-brothers and one step-brother (never-married, deceased).
Now, let's say my step-brother was married and had a child.
1. I sponsor my half-brother;
2. He sponsors his half-brother's child (my step-brother's).
3. He sponsors his half-sibling.
End of story
Even though my step-brother is deceased, his hypothetical children (my nobodies) could immigrate through me.