PeekIt allows you to open, examine, and change (as raw or hex values) the contents of a file. This is very useful for anyone from programmers to people who just want to snoop around the system to learn more.
PeekIt also allows you to annotate files, adding notes about interesting data locations, fields, and other information. This can be a huge boon to programmers working in remote teams, and with the addition of exporting .pinotes files in 2.4 this gets even better, see the Notes section of the docs.
The ability to directly modify the contents of any file can be invaluable, for everything from making a change in a file you don't have an application for, to fixing a value quickly during debugging of an application.
PeekIt can also be a fun tool to explore and learn, and in some cases make other applications more usable. Peeking a game save file, for example, can teach you how programmers think and work when they are saving a complex bunch of information. If along the way you figure out how to give yourself 1,000,000 extra gold pieces that can be fun, too!
It's important to note that PeekIt will make no changes to a file unless you specifically indicate that you want to make changes. PeekIt will never modify a file accidentally or without your complete knowledge. PeekIt also uses very little memory, it does not read the entire file under investigation into memory, only a small portion.