We express gratitude for the pleasure derived from a specific object or circumstance.
I eat His bread, drink His water, enjoy all that comes into my area of experience.
And the blessing creates a certain togetherness, a personal bond with the Divine Source.
In a way, it is very intimate, more meaningful than words, because one understands the slice of bread in a way that one does not understand passages of Torah.
God's speech is beyond me, but His address to me in this slice of bread is something I can grasp.
We can see therefore that the blessing is built around a specific object and involves enjoyment of the senses in order to establish genuine contact.
Thus, too, the tendency is to anchor the marriage blessings in something definite.
As in so much of Halachah, the inclination is to extricate the precept from the abstract and bring it down to the objectively concrete.
Most of the abstract blessings, like those establishing the holiness of a special day or the sanctity of marriage, are accompanied by a blessing on the bread and wine.
The connection with the bread or wine creates a togetherness that is somehow substantial.
But the ultimate significance of these blessings is that they are a way of drawing down Divine influence.
Every blessing first assumes that the power of God prevails over all and then proceeds with its own plea for Divine intervention, that God should "manifest" or somehow take part in this particular circumstance.
–Rabbi Adin Steinsaltz
From “Faith and Prayer,” in In the Beginning by Rabbi Adin Steinsaltz