Heart of the Swarm Campaign- Char/Kaldir: Conclusion

As you have Kerrigan head out from the first planet whose missions you complete, the Leviathan is approached by a Protoss ship- but it does not attack.  Instead, Zeratul arrives in Sarah's chamber.  After a brief bout of psionically-enhanced fisticuffs from the returned Zerg monarch, he explains to her why he is present- he has been to Zerus, the world from which the Zerg originally hail, and on which they continue to fight, devour one another, and evolve.  He insists that Kerrigan must go there and grow from the experience- should she survive it.

This opens up Zerus as a planet to visit, and makes your decisions a bit harder- Zerus' Primal Zerg missions are difficult, but the rewards are immense- not only do you gain access to mutalisks and the new swarm host units here, but Sarah will also gain over twenty levels by progressing through the planet's missions.

You can (and probably should) choose to go to the other of the first two planets (Char or Kaldir) instead- while these missions are notably simpler and slightly easier to complete, it’s more important to be aware of the units that are granted to you.  Hydralisks are of particular importance on Zeras, where you’re going to have to deal with a lot of airborne units from time to time and just possessing ranged attackers makes a big difference dealing with the enemy hydralisks, roaches, and other, more unique, ranged attackers.  At the same time, while banelings aren’t going to be of too much use, aberrations go a long way to counteract enemy ultralisks and make up for your own lack of same- particularly with their ability to literally walk over your other units and thus move from one side of a defending force to the other without any cross-interference between them and your ranged units.

It’s actually possible to leave one of these two planets until nearly the end of the entire campaign and, if you really want to, go ahead and do it- but there’s no actual reason to, and the units you gain access to with these two clusters of missions are very useful immediately and later on down the line.