Behind candle-lighting there stand two laws: honoring the Sabbath and Sabbath enjoyment.
If honoring the Sabbath is enough to arrive at the Jewish law that one must light candles, why do we need to invoke the law of Sabbath enjoyment as well? And vice versa.
So the question is general: when there are two laws and each one by itself is enough, why do we need both?
Maybe the answer is: “What do you mean why? That’s just the reality—that behind that Jewish law stand two laws.” But maybe you have a deeper answer?

This is not a question of depth but of different kinds of “two laws.”
In your example, one can understand that each component by itself is enough to obligate candle-lighting, and one can also understand that you need the combination of both. The second possibility is not really “two laws.” It is one composite law.
In the first possibility, indeed each of the laws by itself is enough to obligate, but there is no difficulty as to why both are needed. That is the whole idea of the two-laws method: that each one on its own, even if it appears without the other, obligates candle-lighting. Applications of the two-laws approach of this type usually deal with situations where there is only honor of the Sabbath without enjoyment (for example, in the case of a blind person), or vice versa (if I honored the Sabbath in some other way and the candles are needed only for enjoyment). Another example is if someone lights candles in his home through an agent. Honor of the Sabbath may perhaps be fulfilled, but the enjoyment has to be my own personally (agency does not apply to things that pertain to one’s own person).
There is much more that could be said about this.