For an invention, a lack of novelty matters for two main reasons:
But even if an idea is novel, novelty on its own may not mean much. For an invention to have good commercial potential, it needs to be a significant improvement on prior art.
Whether or not an invention has a significant improvement depends on many factors. Some improvements may be small in technology terms but have high commercial value.
For example, the drinks can ring-pull is simple technology, but its advantages - (a) it is secured by a rivet that does not penetrate the can, and (b) the underside of the ring is shaped to give mechanical advantage - made it a significant invention with huge commercial value.
On the other hand, it is possible for a commercially successful idea to be novel but not particularly inventive.
For example, electric toothbrushes used to be too expensive to sell well. Then someone discovered that it was possible to use a much cheaper motor. Prices fell and sales soared. This new type of electric toothbrush simply included a well known motor, and functioned in a well known way, so there was no invention - but the novelty of combining motor and toothbrush gave it a large commercial advantage.