OT: sed -n vs. sed

[William Ballard <40414.nospam@comcast.net>]

sed -n seems to make all the other commands sed can do besides 'p' have 
no effect.

$ date | sed 's/ /_/g'
Sat_May_15_14:51:45_PDT_2004

$ date | sed -n 's/ /_/g
[no output at all]

What semantically huge iceberg of a use case am I missing that makes 
"sed -n" useful?