One of the major factors in the wear and tear on outdoor wood is moisture absorption (and subsequent release). This causes swelling and shrinking of fibers and leads to cracks and eventual breakdown of the material.
The end grain is most susceptible to water penetration. Best practice says cover every side of a board with a protective coating, especially the ends. Many builders stick the each end of outdoor wood in a bucket of protective coating and let it sit overnight (obviously one end at a time).
Once installed, it is much harder to coat any surface but the top, so precoating before installation is the better way to go.
There are a wide variety of coatings and sealers. Some have more pigments to mask damage and discoloration on old wood. In new wood, you don't need pigment to mask imperfections. Consider a clear or natural penetrating sealer intended for the particular wood you are using.