The Warsphere is the last vestige of the power and advancement of the Kroot prior to their move away from advanced technology. They are effectively flying towns comprising of a central section primarily housing the main Warp-Engine and outer sections with manoeuvring thrusters. While the engines are reliable, they are also of quite poor power output making Warspheres extremely slow, however they are sufficiently powerful to enable planetary landings. Once on the surface the thrusters are often used to bury the Warsphere, presumably to enable it to revert to its role as a traditional town. Warspheres are fairly rare and so will only be deployed in the case that the Kroot are getting a substantial payment or bonus.
When travelling the Kroot have no difficulty using Warp-Drives unlike the Tau, though how they do this is unknown to the Tau. They appear to have a natural affinity for navigating towards inhabitable planets, almost like a sixth sense.