Why would a company do this?  

Merchants did it to maintain their competitive edge.  This way they avoided revealing their secret sources.  This practice was most common ca 1900 - 1930s. which probably dates this hand painted floral set.  In this case, that was by A Schmidt & Son.

Overmarks are usually from about 1900.  It can be valuable to collectors.  Sometimes these marks contain factory logos, artist names and even a signature. 

This particular set is hand painted and being offered for sale by www.AprilsLuxuries.Etsy.com and here on www.AprilsLuxuries.com