What is the difference between Oyster card pay as you go and a season ticket (Bus & Tram pass or Travelcard)?
Using pay as you go involves an amount of risk. That is, you can never say for certain that touching in and out will be faultless. Every time it is not it will cost you £4.00. Nor can you guarantee that the gates will work properly or that you will be correctly informed by staff on how to deal with certain difficulties. Apart from this, pay as you go is a lot cheaper for people who just travel occasionally.