Ideally, provenance gives you the history all the way to the origin or early owner, but, for altered cards, even just going back a step or two can reveal issues. If you find an earlier auction showing that a card has been altered, that is an example of provenance and why it is important. Game used collectors often find where a bat or jersey has been altered since it's last sale (number on a bat change, fake use added, etc) -- which is an example of provenance. The linked column at the bottom gives examples where past sales have shown problems with items.
Of course, provenance isn't foolproof and doesn't prove everything. Of course, there isn't documentation and pictures for all cards. Of course, some cards are obtained from a garage sale or found in an attic. Even if someone has a receipt from 30 years ago, there probably isn't a picture. But it still an important consideration and piece in the puzzle. If a card doctor is offering a Gem Mint graded card, he won't be able to show where he got the card-- because he didn't obtain it in that state. If someone is about to drop $50,000 on a Gem Mint card and doesn't even ask where the card came from, I would say that person is pretty stupid.
In fact, I bet in the future, cards with decent provenance-- as opposed to high grade cards that appear out of nowhere-- will receive a premium in value. Many card collects ignore or dismiss provenance for cards, but if it is revealed that there has been mass doctoring of cards, they likely will change their tune.
With famous old masters artworks, provenance helps prove authenticity and identity, along with giving evidence of its age. Here it's not a matter of condition, but identity and age.
The Importance of Provenance in Collecting