Originally Posted by HeathrowGuy
Because it's a document that can be brought into court and readily used against a foreigner for perjury/false declarations to a government official if the need so arises.
I think I'll need more information here. I have a very hard time remembering the last time I lied on an immigration card. I do check "honeymoon" for "purpose of trip" every time I fly to Hawaii, and I couldn't resist selecting "agriculteur/pecheur" for my profession on my last Tahiti trip, but I don't think people so routinely make false declarations on their immigration cards that these documents provide significant backup for detentions and the like.