¿Debo hacer una copia de seguridad de las aplicaciones en itunes?

Update octubre 2017: As of iTunes 12.7, the entire concept of backed-up apps has been removed from iTunes, so you should no longer see any prompts about backing up apps when syncing or backing up in iTunes. Apple is supporting a version of iTunes through early iOS 12 that supports a Mac/PC iOS app library and installing apps through iTunes but this version of iTunes is not compatible with macOS Mojave, nor do we expect it to continue to be supported moving forward.

Here's the original post information, should you need it for iTunes or earlier.

Si hace clic en el botón Copia de seguridad en itunes, lo más probable es que vea el siguiente cuadro de diálogo:

There are apps on Kelly's iPhone 5s which are not in your iTunes library. Would you like to back up these apps from your iPhone?

¿Qué significa esto? Y, como todos los cuadros de diálogo que itunes me muestra, ¿debería hacer clic en el botón no como me gusta? :)

Respuesta corta: Sí, debe realizar una copia de seguridad de sus aplicaciones en itunes.

Long answer: As part of the normal iPhone backup, your data saved within each of the apps is backed up, but not the actual application itself. Backing up apps in iTunes means that iTunes is going to copy the application themselves (not your data, but the files that make the app run) to your iTunes library. This used to be critical before you could download past purchases from iCloud on your device, but it is still useful. If you later need to wipe and restore your iPhone from a backup, iTunes will install your apps back onto your restored phone, but only those apps backed up in iTunes. For the apps not backed-up in iTunes, you'll have to remember what apps you had and download them from the app store (and/or iCloud) again. (Edit: as of iOS 9, the apps that were installed at the time the backup was made are downloaded from the App Store and reinstalled after restoring the backup if you sign in with the same Apple ID during setup.)

Si eliges no hacer una copia de seguridad de tus aplicaciones en itunes, el proceso de restauración aún pondrá tus datos nuevamente en el iPhone para esas aplicaciones, simplemente no instalará la aplicación por ti. Una vez que descargue la aplicación y la abra, sus datos estarán allí esperando por usted.

If the idea of remembering what apps you had installed and manually installing each of them after a restore sounds tiring, then yes, you should choose to back up your apps in iTunes. But, if you're always running out of disk space on your Mac/PC, you may want to trade the possibility for future hassle for precious megabytes now.