I have to admit that I've spent hours looking at trailers over the past couple of years, some people certainly put quite some thought into the design. I just loved the idea of not having to hoist the kayak onto the roof of the car, avoiding potential damage to myself and the car. The bigger positive is not having my car packed full of gear. It's not a big issue if there's only me on a day trip, though if I'm taking someone along for the trip, not having to jam two lots of gear into the car is good news. Then there's wet stinking gear and bait being left in the car overnight... ahhh, that morning smell when you open the doors, plus the piles of sand covering the carpets and upholstery.
It's not a must of course, just a nice to have.
Below is a lightweight trailer a friend knocked up from a motorcycle trailer, simple and effective. What I did learn recently is that loading a kayak upright onto roof bars, is that once strapped down it will deform the hull if not suitably supported. This one was well supported and had no issues.
Here's some more great examples of home built/modified trailers.