Planets are not all created equal: planets may have fueling or repair capabilities, and agrarian worlds produce armies at an accelerated rate. There is also a starbase unit that often functions as a mobile planet with fueling, repair, and refitting capabilities, as well as the ability to accommodate many army units.
Armies may be bombed while they wait to be picked up on friendly worlds, so they soon become a precious resource, and agrarian worlds are thus favorite targets for early conquest.
The ability of a player to transport armies is proportional to the number of enemy kills since the player was last killed. Players with many kills are called carriers, and are valuable resources.
Finally, position is very important. When a player is killed they reincarnate near their homeworld. This becomes important as the front moves closer to one of the homeworlds. Position is also important on a smaller scale, and experienced players will often use tractor and pressor beams to relocate enemy units. Crippling an enemy far from an active area is also a type of positioning strategy (for example, sending a fresh ship to the enemy homeworld may be undesirable).