Butterflies are beautiful, colorful bonus of our garden.

Butterfly lays eggs on the backside of the plant. Every butterfly species has specific host plants. Caterpillar comes out of the egg. It is greedy eater. It eats leaves. That's how we usually discover their presence, by partially-eaten leaves. Eventually it enters pupa stage. Caterpillar's greedy eating is preparation for this stage. Finally, one day butterfly emerges out of pupa. Its wings are wet and folded. It expands them and waits till they are dry, before flying away.
They do not harm plants. Even if plant loses a few leaves, it is fine. Please do not kill them. In fact, make it a point to have a few host plants in your garden. In cities, butterflies are losing habitat. Let's create a place for them in our garden.