Better beams can drain and dry
The typical way to build a deck beam is not the best way. Normally, two or three 2x8s or 2x10s are nailed together with 16d spikes, three or four every couple of feet.
This is a plenty strong way to build a beam, but it can trap moisture and crud (making moist crud) along the top where the 2x8s meet. Because the edges are rounded slightly, this joint creates a groove in the top of the beam, which traps water and remains damp for a long time after a rain.
Damp beams = #BugsMoldRot.
The better way to build a beam is to use half-inch spacers between the layers. The space allows drainage and airflow.
In this video, Steve Maxwell illustrates a good way to make the spacers: pressure-treated plywood with pointed tops to shed water. He recommends using carriage bolts, rather than nails, to draw the beam together.