Why do I need to know c’est ça ?

Because it's a common and versatile expression, often used in a way  that your French teacher won't teach you.

What does it mean ?

Literally, c’est ça translates to ‘that’s it’, in the sense that you got something right.

For example, if you’re asking for confirmation that you have understood something correctly, and the person you're asking answers c’est ça, it means 'exactly' or 'that's right'.

C'est ça here takes the same meaning as voilà, so 'you got it', 'there you go'.

Beware, however, that c'est ça is also widely used to express sarcasm or skepticism, just like you'd use 'right' or 'sure' in English.

If somebody tells you they're a secret member of the British royal family, you could answer, ouais, c’est ça !, your tone dripping of sarcasm.

If your boss tells you that they expect you to answer emails at 3am on Sundays, you could jokingly say c’est ça. (If the boss doesn't take that well and you realise that he or she was serious, you might want to think about finding a different job).

Also, if someone tells a dumb joke and you cannot master to produce a laugh, just say, hehe, c'est ça..

If you’re not sure about the way a French person is using c’est ça while talking to you, it's all in the tone.

Use it like this

J’ai entendu dire que tes parents habitaient dans un château dans le sud de la France. Oui, c’est ça ! – I heard that your parents were living in a castle in the south of France. Yes, right!

Je te jure que je ne t’ai jamais trompé. C’est ça, oui.. – I swear I’ve never cheated on you. Yes, right…

Est-ce que tu pourrais garder les enfants, faire un gâteau, ranger la maison et faire le ménage ? C’est ça oui ! – Could you take care of the kids, bake a cake, tidy the house and clean it ? Sure!

Synonyms

Exactement – Exactly

Bien sûr  – Of course

Mais oui – Yeah sure