Logging data on an Arduino is very much a trivial every-day task. Connect an SD card, open a file, and start printing data to it.
For many people that is good enough. It results in nice easily readable (by us humans) data.
But it's not fast. It's not efficient. It's perfectly fine for things like logging temperature every hour, or barometric pressure every 5 minutes, etc. But when you have large amounts of data to store very rapidly you have to think a little differently.