In article , Alfred
Molon wrote:

What is the second card slot needed for? My camera has two card
slots, but I've been using only one for the past years.

They can be configured in different ways. You can have double the storage,
one
for RAW the other for JPEG or, as preferred by professionals, one will be a
backup of the other.

But is all that really needed, given that there are huge memory
cards (1TB) and the professional will backup the images on the
computer anyway?

the backup protects against card failure.

if you're writing to one card and it fails, you won't have any images
to back up on the computer.

with a second backup card, you will.

it's basically a raid-1.