When you can use Google Music to store 20000 songs, Google+ Photos to store an unlimited number of photos up to 2048 x 2048 pixels and videos up to 15 minutes, YouTube to store all your videos, Gmail to store 10GB of messages and attachments and when the files converted to the Google Drive formats don’t count towards the Google Drive storage, what are the use cases for the Google Drive storage and why would you buy extra storage?
You could use it to store the original photos, PDF files, store documents/spreadsheets/presentations without importing them into Google Docs, backup all your files, but the most popular file types are already covered. Google wanted to encourage people to use Gmail, Google Docs, YouTube, Google Music, Google+ and free storage was an important selling point.
The trouble with Google Drive is that Google has a lot of other services that are better suited for storing your files and Drive doesn’t even integrate with those services. When your photos are in Google+ Photos, your videos are uploaded to YouTube and your music is in Google Music, Google Drive feels like an afterthought, not the “go-to place” for uploading your files.