Primary, Secondary, Tertiary and so on are used by many polyamorists because it makes logical sense or is in some way desirable to have a relationship hierarchy or acknowledge the significance of relationship levels (based on whatever the paramours wish to base levels on).
There are also some polyamorists who strongly dislike these terms and try very hard to create lateral only relationships where equal significance is given to each relationship to the extent possible.
And then --of corse (since it is polyamory we're talking about here)-- there are times when one kind falls in love with the other kind and a network will develop that has some of each. At which point there is a wrestling match, or somebody gets voted off the island, or... I don't have my rule book handy.