You are correct. A long form can be many combo sequences that put together. Since there is no logic connection between each combo sequence. It makes no sense to train the whole form. It makes better sense to train just the combo sequence.
Alot of forms (particularly in karate) actually do have a theme or principle that links through the whole thing and that it's trying to teach. Even the particular names given to the form will often allude to the principle that runs through the form.
(Some random notes on my phone haha) --->
For example
The kata "Seiunchin", a few translations are "the calm in the storm", and "grab and pull in battle"
Close in, wrestling, grabbing
Elephant
The strength of the elephant tusks, strong immovable. That AIN'T moving. Decide, I am NOT moving. NOTHING can move me. Take on that attitude.
This mindset helps strengthen the body.
Twisting action
Enormous pulling and pushing power
FEEL the strength of it, EMBODY that