You can use "I guess..." at the start of your answer to a question for a few reasons:

• to show that you're not sure of your answer
• to show that you just thought of the answer after you were asked the question
• to show that you're thinking carefully about your answer, and not just repeating an answer that you've given before

So it's a good idea to start your answer with "I guess" when someone asks you a personal question about why you made a certain decision. There are also other reasons for using "I guess".