Sign up for any of Google's online services and you get 15GB of storage space for free. This is used for several different things, such as all your emails and their attachments at Gmail. There is Google Drive itself and you may have documents, PDFs and other files stored in your online drive. They all come out of your 15GB allowance.
If you have an mobile phone and it is set to automatically store your photographs online, then they consume your 15GB allowance too. If you take a lot of high resolution photos you could easily find yourself running short of storage space. (A little known fact is that if you shoot photos at less than 2048 x 2048 pixels then they don't count as using any storage space.)
Upgrading to 100GB of storage space used to cost $4.99 a month, but this has been slashed to $1.99, and 1TB of storage has been reduced from $49.99 a month to just $9.99. Now that's cheap!