Smoke had an imprecise shape and was not comparable to other physical objects, represents the religious feeling and experience. Both were offered at the same time in the morning and evening (T'zaveh 30:7-8), since both form crucial components of the framework of our avodas Hashem.
To quote the powerful, eloquent words of the Rav:
What does ketores represent? The hidden and the intimate the mysterium magnum of creation and the mysterium tremendum of Divine presence in creation and beyond. ... Ketores tells us the great story of human craving for G-d ...for the Beginning of All... of tragic humans waiting for an ecstatic unity with the Almighty. The colorful religious experience is represented by ketores. The clear, intelligent religious fact and intelligent action is represented by the menorah.
The Torah, however, admonished us that the burning of the incense be coordinated with the lighting: The mystery feeling with the clarity of thinking and acting; the excitement and passion of craving be united with the serenity and peace of halachic comprehension and halachic implementation. Both are necessary. ... Yet ketones cannot be separated from hadlakas neiros. The subjective must never be isolated from the objective. ... the halachic detail and the halachic precision is necessary for one who wants to obtain a great, colorful religious experience."
