Often we forget that plants bloom to breed. That is their reason to
make a flower.

A plant that is fragrant is trying to attract a pollenator. B. nodosa
is fragrant at night because that is when the moth that pollenates the
flower is active. The white color and the odor is what attracts the
critter that is necessary to carry the pollen to the next plant.

Generally, white flowers are pollenated at night and the white color
enhances the ability for the critter to find the flower. A spot on the
throat of the flower makes a nice target for the pollenator to aim for.

Some orchids mimic a female bee (Ophrys) that when the drunken male bee
comes home after a hard night at the local bar, his eyesight is a bit
off. He sees this flower that looks like a sweet female and tries to
make love to her. Frustrated he moves to the next "female" and
continues until he gets fed up and goes home. His wife finds the pollen
and his collar and knows where he's been.

With this cattleya you should research the species parents and figure
out why it would be fragrant. Of course the more the genes get mixed up
the plant may be confused and may not mimic the species.

I've had some catts that are fragrant only for a brief moment when the
sun first comes up then nothing the rest of the day. Some only when the
sun is on them, some of them only when the air is warm, etc. It's all
genetic.

Good growing,
Gene