This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
As you can see from the above chart, travelling at integral Warp
factors is much more energy efficient. But there are times when a
fractional value must be used - for example, staying a certain
distance from another ship, or keeping pace with some phenomenon.
Also, beyond Warp 9, only fractional speeds are possible. (Modulo "All
Good Things..." [TNG], of course.)
 
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