Every Thursday Apple reveals a new free app of the week. Some apps are likely to be used a lot, such as last week's free app, 360 Panorama. Other apps, like this week's Kayak Pro, may not sound like something you're going to use today, or even in the near future, but you should download it anyway. Why? As pointed out on Reddit, and then by The Next Web -- because it's free.
If you're not going to use the app right away, delete it from your iOS device. There's no point in letting it take up space on your device, and you can always download it again in the future without having to pay for it. And, thanks to iCloud and the purchased section of the App Store, you can easily find apps you've previously purchased and download them in just a few taps.
If you end up needing an app you have downloaded and later deleted from your device, launch the App Store on your iOS device and tap on Updates. At the top there will be a Purchased option; tap on it. Then tap on the Not on this iPhone (or iPad, iPod). You should then see a list of apps you have purchased, but not currently installed on the device you're using. Find the app you want to use, and download it.
The approach to taking advantage of free app promotions is nothing new. Even before Apple rolled out the recent purchase section of the App Store with iCloud, users were able to search the name of an app and re-download it for free; if they could remember the name of the app, that is.
If you see a tweet, or a blog post with a current promotion for a free app, be it Apple's free app of the week, or from a developer, take advantage of it. You never know what kind of app you'll end up needing or using in the future.