For most Mac users the problem is that their OS X accounts aren't really private – we let other people use our machines with our account, or there are other admin users on the same machine (who can see all files). unless you have the FileVault feature enabled (a good idea) it's relatively easy for even an intermediate user to connect to one Mac as an external HD, allowing you to see all files on that machine. If they use the machine under OS X account but one with admin access they still have access to all your files. If they use it under your OS X account they have access to all your files.
There are a few good (reasonable, non-paranoid) reasons you might want to secure information on a Mac, including. Why Would You Want To Hide Your Files On A Mac? Yours would be the only admin account on the machine, only you would use that account, and you'd be using the FileVault option. In a perfect world securing the files on a Mac would be easy. How to hide/secure files on your Mac: An easy approach to protecting your data by securing, hiding, and encrypting selected files and folders in Mac OS X. How To Securely Hide (and Encrypt) Files On Mac OS X Oct 03, 2016
