You don't have to buy videos as you can use the credit to rent movies, tv episodes, etc. The Instant Video credit can be used on the 24 hour VOD rentals from Instant Video that include New Releases.
Some people also seem to have the misconception that Prime members get access to the entire Instant Video offerings including New Releases, or that the Instant Video Credit can ONLY be used on the limited selection of Prime Instant Video titles.
The $10 Instant Vidoe credit can be used on EVERYTHING in their online video offerings! Whether you want to use it towards the purchase of a $14.99 New Release title or rent a New Release for $3.99 for 24 hours. I plan on renting 2 New Releases movies and using the remainder to rent a couple of $0.99 movies that are on sale.
I could rent a new release DVD from redbox for a day and it'd cost me $1.20 if I didn't have all these free rental codes, $1.50 if I want blu-ray. Face it, compared to newegg's occasional $12-15 off the price offerings and MS store's $25 gift card, amazon's become a joke.
Next time I will pay better attention to the pre-order bonus. I could have gotten a $10 GC from Target or $12 OFF AT NewEgg. As I have to pay tax on Amazon, I would have gotten a better deal elsewhere.