I think primary denotes an ultimate level of commitment such as marriage. When you are that committed to someone it tends to involve more than just emotional support. It usually involves finances and a domestic partnership, maybe children, etc. It is in one’s best interest to nurture and preserve the primary relationship, even at the expense of secondary relationships if needed, (so long as the primary is still a healthy and viable option, sometimes you just need a divorce). Ending a primary relationship tends to majorly disrupt one’s life in more than emotional ways.
This does not mean that one cares for secondaries any less, or thinks of them as less important of a person. It just denotes levels of commitment. In fact I think most secondary relationships go better if both of their primary relationships are healthy.
I don’t think everyone needs to use these terms, as not everyone truly has a primary or wishes to depend on another for non- sexual/emotional life needs.