This could be the largest single profit center for cruise lines today. They will arrange shore excursions for you, but the price is often far greater than what you can arrange by simply stepping off the ship at the port and making a few inquiries.
If you're someone who doesn't want to carry cash or negotiate with drivers or tour operations, perhaps paying the cruise line to do it for you is acceptable. But some of the mark-ups are ridiculous.
Read more about how overpriced shore excursions could put a dent in your cruise budget.