Why Copepods sold with Phytoplankton?

[Obriy]

I've learned from different sources that copepods are consuming diatoms and it is good to add pods after a new tank cycled and diatoms started growing in a tank.
When shopping for copepods, I see that they are often paired with Phytoplankton because Phytoplankton is copepods' food.
Question - do I really need Phytoplankton at this stage? Because, if the pods are fed, why would they want to consume diatoms? I have no corals yet, so coral food isn't needed.

 

[ZzyzxRiver]

The phyto has other benefits, but for the copepods specifically it helps the larva in their free-floating stage. If you want to accelerate their reproduction, phyto is the way to go