The alternatives to pre-sales chat are email and phone support. Email takes too long. You want to respond to a customer's questions while that customer is still in the mood to buy. Also, email isn't interactive. If the customer has more questions or if you need more information to answer the question, then the dialog can drag on.
Phone support is expensive and annoying.
Only about a fifth of Americans have more than one phone line in their homes. This means that they have to disconnect from the Web to make the call. It is far more expensive to acquire a new customer than to entice an existing customer to return. It's important to create loyal customers because you cannot afford to acquire enough customers to be profitable. It often takes three or more purchases by the same customer to recover the cost of acquisition of that customer.