You don't make binary files to get garbage in Notepad. The reason for binary files is that you can use datatypes such as Long, Integer, Byte and others for a specific kinds of uses. Sometimes data isn't even in an easy format, instead you have to read a bit of the file according to a specification just to know how to continue reading the file correctly (for example, ID3v2 tags in MP3 files, and the MP3 files themselves - ID3v2 just has readable text in the tag and not just binary data).
A simple file could be:
1000 32-bit values (Long)
These values contain a byte position to a starting point of a string.
After the 1000 values (which take 4000 bytes) the string data begins.
Each string ends with a character 0 (nullchar).
That would be a pretty minimal file format. Something similar was used in the first Command & Conquer series of games.