Downloading and storing images/data locally

I'm rather new in coding with API's so I was wondering if someone could clarify somethings for me.


NOTE ON IMAGES: Do not continually hotlink images directly from this site. Please download and re-host the images yourself. Failure to do so will result in an IP blacklist. Please read this guide on where to download images.

Please download and store all data pulled from this API locally to keep the amount of API calls used to a minimum. Failure to do so may result in either your IP address being blacklisted or the API being rolled back.

So all data I pull, I should be storing locally after every pull and just use what is stored instead of having a duplicate API call later on. So why would I even access the API if I can just pull all the data, one time, all at once and just store that and access that and only ever update the stored data when new cards are added or a different change has been made?

I assume I'm wrong in my assumptions, but that's why I would like someone to clarify it for me, because If I should only pull every bit of data once, why would I not bulk download it all and then never access the API again except for updating my stored data?