Delete data you no longer need. Data always carry the risk of being abused. Therefore, you should protect data that you still use and permanently delete unnecessary data.
A common problem are USB flash drives. These often include a large number of private and intimate data. If you get asked by someone to hand off your USB flash drive to fulfill a good purpose, this person can be able to recreate even your deleted data without any problems.
The standard delete function of the most operating systems is bad and inadequate. In most cases it only deletes the file system's file entry rather than overwriting the file with zeros, or pseudo-random patterns. This is comparable to tearing out the first page (the table of contents) of a book: the pages are still there, but they can no longer be found easily.
Thus it is very easy to find and recreate even "deleted" files from your flash disk. The "finder" would now be in possession of your most private data. That opens all doors for misuse.
Intimate details such as photos are still a minor problem. Wrong deleted (or undeleted) data could facilitate spying or make it possible at all.
Erase your data!