The deities of the realms are powerful but far from omnipotent, knowledgeable but far from omniscient, widely traveled but far from omnipresent. They alone of all creatures in the universe consist only of astral essence. The gods are creatures of thought and ideal, not bound by the same limitations as beings of flesh.
Because of their astral nature, the gods can perform deeds that physical creatures can’t. They can appear in the minds of other creatures, speaking to them in dreams or visions, without being present in physical form. They can appear in multiple places at once. They can listen to the prayers of their followers (but they don’t always). But they can also make physical forms for themselves with a moment’s effort, and they do when the need arises—when presumptuous epic-level mortal adventurers dare to challenge them in their own dominions, for example. In these forms, they can fight and be fought, and they can suffer terrible consequences as a result. However, to destroy a god requires more than merely striking its physical form down with spell or sword. Gods have killed other gods (Asmodeus being the first to do so), and the primordials killed many gods during their great war. For a mortal to accomplish this deed would require rituals of awesome power to bind a god to its physical form—and then a truly epic battle to defeat that form.
The most powerful servants of the gods are their exarchs. Some exarchs are angels whose faithful service has earned them this exalted status. Others were once mortal servants who won the station through their mighty deeds. Asmodeus has devils as exarchs, and both Bahamut and Tiamat have granted that status to powerful dragons. Every exarch is a unique example of its kind, empowered with capabilities far beyond those of other angels, mortals, or monsters.