An experienced player will know how important it is to get a healthy economy. With a healthy economy comes plenty of resources, and with plenty of resources comes plenty of possibilities for the player, such as building more gateways or robotics facilities, building more fighting units, building structures required to have more powerful units, and so on. Therefore, the primary concern of the Protoss is how aggressively they can expand (build new nexuses with probes mining minerals and gas) and how well they can protect these expansions. Without a healthy economy, a player is toast. Always.