A lighter boat displaces less water as it moves, making it easier to move through the water. It is going to make a boat more responsive to each input of the paddler. Stiffness is a factor to consider for longer touring kayaks. A stiffer boat is a more efficient boat. If you paddle a boat that flexes under you as you paddle, that flexing is using energy that you would rather be using to propel you forward.
Durability is a very important factor for most kayakers.
Nobody wants to invest a lot of money in something that won't last. Ideally you want a boat that you can accidentally drop off of your roof rack or bash into a submerged rock without worry. It goes without saying that cost is a factor in everyone's buying decision, but depending on what you intend to use your boat for, you may find that a less expensive boat is better for you. Now we can look at the different hull materials builders are using and why. Rotomolded polyethylene has become the most popular material as technology has led manufacturers to stiffer and more durable plastic make-ups.