Programs were encoded using the cards but because each card (if using the IBM 80 column card method) could only hold one line of code then you might need hundreds of cards for a serious program. All the cards had to be in order and there are plenty of tales of chaos caused by people dropping card stacks!
One benefit of this data storage method however was that if you did suffer such a catastrophic data corruption you could restore it by putting the cards back in order! Corrupted cards (bent for example) could also repunched and then replace the defective card.
By the 1970s computers were moving onto magnetic storage and visual display terminals though you could still find the keypunches and readers well into the 1990s. I remember at university in the early 1990s one room still had an IBM keypunch machine - though it was not in use. We used the far more up to date Volker-Craig dumb terminals to type our programs instead!
