Why do I need to add logic to a survey question?

You can add logic to a question to create a dependency between questions. Below is an example. 

Let us add three questions - Q1, Q2, and Q3 to a survey. 

Q1: Which Grasspods product do you use?

1. Speakers
2. Home Theatre
3. Headphones
4. Projectors

Q2: How is the sound quality of the headphones?

1. Excellent
2. Good
3. Average
4. Below Average
5. Poor

Q3: How is the noise cancellation of the headphones?

1. Ordinary
2. Good
3. Amazing
4. Bad

In the case above, we can add logic to Q1 as follows.
If the answer Equal To Headphones 
Show Q2 and Q3
Similarly, you can add logic to the other questions.