Justification I’ve heard is that if one part of the couple is managing the other, or is promoted after the relationship started, then:

• there is a power imbalance in the couple, possibly one is coercing the other (« I can’t leave him/her, they’ll make my worklife hell / get me fired »);
• there is a risk the manager will promote their partner even if their job performance doesn’t warrant it

Companies will want to both avoid this sort of things, and avoid being seen to enable this sort of things.

I’ve once worked at a company that wanted to know about relationships between their employees and suppliers/customers’ employees, again because that might enable situations where a supplier / customer is treater favourably because of personal relationships