You do not need to book a tour with a travel agency to see the northern lights. If the sky is clear and the auroral activity is good, you can see northern lights even from the city center of Svolvær.
However, booking a tour with an experienced guide is always better to increase your chance of seeing the northern lights.
On cloudy days, the guides are excellent at reading the local weather forecast and choosing the spot with the most significant chance of a hole in the cloud cover.
On top of that, you will learn a lot about the region, local legends, and the northern lights. The guides will also take pictures of you and the lights, which requires some practice, and give tips on taking great pictures with your own camera.