Residence issue. HK residence? Mainland residence? For emmigration, family reunion? Apply seperately? Or togather?
Tax calculation. It can be file togather to have the max tax reduction!
Social welfare. Same as above.
Inheritence. Spouse has a priority.
Insurance and contractual issue. Spouse has an absolute privelege. Anything can be said without fear of libel or slander. In contract, both person is view "somewhat" as one in that the they can somewhat represent each others.
And on and on and on....
In the case of divorce,
Sharing of wealth