Packing more copy in a narrow space is the main reason for using a condensed type. Characters with a more ovular shape tend to be less wide than their circular counterparts and will allow for more letters per line. In narrow columns for example, this typeface can provide up to 25% more copy than the regular typeface in the same space.
Another reason is when a larger type size is called for — used sparingly it is useful for headings or headlines.
For emphasis, narrower letters can provide a stark contrast in the flow of reading, creating impact while retaining typographic character. Condensed types can specially useful in tables and charts because typically both use few words in each block. If space now allows, you may think about the luxury of a larger point size. This optimizes space while keeping your typography more easily legible.
