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. They might want to move one of the parties to a different department so that the higher up one doesn't make promotion decisions for the other.
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
And Fabrice Bellard, the original author of ffmpeg, went on to create qemu which pretty much made open-source virtualization possible. Also TCC (even if I don't think that one is widely used), he established a world record for computing decimals of Pi using a single machine that had ~2000× less FLOPS than the previous record, and so much more...