At site redesign, the question does arise for the content provider of an already exisiting site?

A simple answer would be : “Fresh content is good, especially when it is good content”. But Old Content is part of your history in SE index; it acts as some sort of foundation. Build on it. Add some new on top of the old. Augment!
In conclusion, keeping the old content in place and adding new one on a regular basis would leverage legacy and freshness with both your visitors and the search engines.

In editorial copy.
The fundamental building blocks of information are words. Alone, they encode meaning and weaved together, they transmit ideas. But all words are not made equal.

Stop Words (of, as, by, …) are extremely common words that are automatically excluded by the search engine during a query unless otherwise included by using boolean operators.

Keywords, on the contrary, count more toward comprehension in communication. As they are relatively in smaller number than others words, keywords concentrate concepts and henceforth, carry traffic.
Search engines are looking for them in editorial content, anchor links and titles to aggregate websites into tighly focused categories.

In search results
After being visited by the search engines and based on context retrieved from its editorial content, a website is assigned a relevancy value given in comparaison to already known websites on the same topic.
This perceived pertinence translates into a rank in the search engines database.

Note: Modifiers as verbs, adjectives, numbers, etc. tend to deteriorate the quality of search results by adding too much variables in the query. Instead, aggregating more than 2 keywords together and using Boolean operators (AND, OR, NOT, +, -, …) results in better targeted searches.