Poker is a game of situations, and for every situation there will be a correct way to act. You learn these correct ways to act through a variety of different forums. Some of these forums include playing experience, talking poker with other poker players, reading books and getting coaching.
These situations occur repeatedly in Texas holdem poker and it is because of the repetitive nature of the events that it makes sense to create your own system for playing the game. Have you ever heard the term playing on autopilot? Players who operate in this manner do so because they have a system.
The system has been created and refined for so long that it becomes second nature to them, unlike a beginner who has only just began playing their poker download and therefore have to think about their every move that little but more. Having a system that allows people to play on autopilot also frees up brain capacity. Freeing up brain capacity when playing poker is vital because it allows you to think about the more complicated problems (especially when multi-tabling online) for example.
The last reason having a poker system is very important is for continuous improvement. If you have a system then you have rules and part of your system should be reviewing your game. Having a specific set of rules allows you to dissect them during review. Rules that are losing you money can be changed and new ones created. When you introduce new rules, because you have specifically done this, you can then measure the success or failure of these rules and this is known as continuous improvement.
Lastly right your system down on a piece of paper. Transferring the rules and processes from your mind and onto paper will really help you improve your game by getting to know your system more intimately.