Store bought cards are plentiful and sometimes can even convey your message. However, creating a hand-made card allows you to personalize your message, choose your own images, be creative, and the recipient will definitely appreciate your efforts a whole lot more!
It may sometimes depend on the purpose of the cards and the number of recipients. For just a few number of recipients, DIY may be a good option. But when it comes to like mass production, for better productivity, you may need to purchase already available templates or have them printed.
That is true. I make numerous custom products and many are personalized. Depending on how many I am making I will use a professional printer to print, mostly because it costs less than the ink cartridges used in my color printer and it can be quicker when printing a large quantity. Sometimes I use both -- the printer for quantity and my inkjet to personalize.