Platform Encryption will go further than just preventing the users from seeing data, it will also encrypt the fields at the database level. For example, you can't even issue a SoQL query doing an "Order By XXX" if that field is encrypted.
Also, PE lets you encrypt fields not subject to FLS (Account and Contact name, for example). In those cases, users will see "****" instead of the actual value in RLs, Reports, Lookups and other things. This also means users might be able to create accounts, but then can't edit them or report on them.
I can find more documentation if the couple of blurbs above do not properly address your question.
Two more things I forgot to mention are
1) Compliance. Platform Encryption is compliant with a number of standards that need stricter restrictions than FLS, or that require that even Sys Admins are shielded from seeing some data.
2) You an encrypt Encrypted Attachments, Files, and Content, which FLS may or may not help you secure.
More resources here and here