The gemara (Ketubot 4a) and poskim (Shulchan Aruch, Yoreh Deah 193:1) refer to the first time a couple having marital relations as "beilas mitzvah" (mitzvah of marital relations).
Being as the couple is obviously not being mekadesh via this biah (see Yevamos 52a, and Rambam, Hilchos Issurei Biah 21:14)-- what exactly is the mitzvah being fulfilled in this biah rishonah that it's referred to as "beilas mitzvah?"