Let's consider two examples.
You work alone at home. Your spouse comes back from outside, and casually shouts :
- Chéri, tu es là ?
- Oui, je suis là.
- Mais où es-tu ?
- Je suis ici.
Your spouse asks whether you're at home or not. Your first reply means that you're home indeed, while the second one refers to your specific location — let's say, the kitchen.
A friend happens to be in your neighbourhood and wants to drop by. He phones you and asks:
- Tu es là aujourd'hui ?
- Non, je ne suis pas là.
Your friend asks if you're at home, and you answer negatively: maybe you're at work, or gone shopping, or taking a walk.
« Être là » needn't refer to home. It could be any implicit location. In the first dialogue, your wife obviously wants to know if you're at home ; so does your friend.
If you're often on the road for professional reasons, a colleague may ask:
- Tu es là aujourd'hui ?
Meaning : are you at the office ? The implicit location is the office.