The average person can only keep 7 (plus or minus 2) items of information in their working memory at a time.
Chunk content in groups of 5-9 in order to not overload the users short term memory.
The application of Miller's Law is especially important for first time users of your website / product.
We naturally group content everyday. For example, we chunk the digits in phone numbers into groups (i.e. 07700-123-456).
Top level navigation
If you want a user to remember all your top level navigation options, limit them down to less than 7 choices.
Chunked content is easier to scan
It is important to chuck information into groups of related content. Chunking large paragraphs of text can also make it much easier for the user to scan for the information they are looking for.