Quality. There's many different travel suppliers - ocean cruise lines, river cruise lines, tour companies, hotels, etc. Once you book a vacation, a lot of your experience rides on their performance. Using a quality travel supplier will go a long way to ensuring a great vacation experience.
Would you hand over a $1000 bill to someone you've never met with expectation that you will get something in return? Can you trust your travel agent? Can you trust the underlying travel supplier? After all, you may be spending between $5000 and $20,000 on a vacation so asking about trust seems pretty reasonable.