Lenticels are striations, horizontal markings on the exterior of the bark, which is composed of dead periderm cells. Trees are living plant organisms, and as such, have a living interior. As is the case with most living organisms, there is a gas exchange that must be facilitated as the tree conducts photosynthesis, the process that supplies plant cells with the energy they need to survive. In the living cells of the tree, primarily the leaves, the stomata open and close to help facilitate this exchange of gasses and water vapor. As previously mentioned, however, there is a need for the trunk of the tree to "breathe" as well. This is where the lenticels come in; since the exterior of the tree bark is dead, the lenticels serve as permanent openings through which the gaseous exchange may occur. Lenticels may be observed as striated openings, usually horizontal to the ground, that are part of the natural appearance of the bark. The resource link I provided below has several good pictures of different tree barks and how the lenticels appear on those trees.
Lenticels are present on the woody dead bark of the plant as it is not subject to turgidity/flaccidity of guard cells. The new epidemis of the stem is still a living tissue, richly supplied with the vascular bundle, especially xylem. Therefore stoma is present on the epidermis because it can be opened/closed as per the plant's requirement.
Dead bark protects the mechanical structure of a plant. But since it is dead cells, the stomata cannot open when needed (gaseous exchange, tranpiration) by the meristematic tissue lying underneath. So to enable the gaseous exchange, transpiration, the bark has lenticels that never close.