Why do companies sell blu ray and DVD combo pack?

Most households that own a Bluray disc player normally have at least one DVD player elsewhere in the house.

It is likely that publishers have decided that offering a DVD with each Bluray disc adds value to the buyer so that the same movie can be watched in the living room in HD and also perhaps in a bedroom that doesn't have access to a Bluray player just yet.

Profit on disc sales is linked far more to the number of sales than the cost of producing the discs themselves. A DVD costs almost nothing to manufacture so the additional cost of the product hardly changes. The increase in sales will certainly outweigh the increase in costs.