Cloud Storage (Google Drive, Box, OneDrive, etc.) is imperative if you have the 16GB Nexus 5 or even for the 32GB if you have lots of photos/music. If you like to listen to music, you should have a cloud streaming service (Google Play music or AmazonMp3 to listen to your own collection or pay for Spotify/Beats/Google Play to stream just about anything).
The definition of "useful" app is relative to how you use your phone, however. Some people text a lot, some play games, some do actual work on it and others use it as a media consumption device.
Waze is a good one if you drive a lot.