the purpose of circuit card?

Answer 1

I interpret your question as the bare, empty, unpopulated circuit board. Its purpose is 2 - fold:
1) A means of mechanically hold the electronic components
2) A means of connecting the components together, in this case called tracks (instead of wires).

Answer 2

I assume you're talking about a circuit board or PCB (printed circuit board)
If so, a CB or PCB (same thing) is for performing a function-any function-and it can be automatic or manual--two examples, (1) a thermostat-you push the temp button up or down and the CB performs the function or (2) the CB in your car runs the electronics on the car and if it fail the car will not fun or your computer MB (mother board) that is the main board for the computer or the graphics board or video board your cell phone/house phone-all circuit boards.