It is one of the docs na dapat meron ka in case you get flagged down by any law enforcement personnel. Let's say nahuli ka dahil may violation ka or na-spot check ka lang ng pulis/LTO.
Why do they ask for it? - It is one of the docs required for the initial registration of a brand new car. So it serves as proof na nasimulan na (or sisimulan pa lang) ng dealer ang registration process.
Other docs required for initial registration that should also be in your possession are the sales invoice and the CTPL (back read a little for CTPL).
After 7days, the CSR and the sales invoice should be replaced by the OR/CR. But this almost never happens. Mine took 15 days bago dumating ang OR/CR. Some friends got theirs after more than a month.