Shinigami, Angels, Death Gods, Soul Reapers, there are many names by which these guardians of the afterlife are known. But names aside, their purpose is the same, escorting souls to their appropriate after-life by the power of the Zanpakuto. Nothing is ever easy, however, and souls that are not reached in time transform into monstrous hollows, which must then be slain and purified.
In ages past, Shinigami did not exist, their task of sending departed souls to the afterlife fulfilled automatically by the sheer presence of the Spirit King in Soul Society. The development and subsequent revolt of the demons changed all that, however, with the Spirit King sealing himself away and giving his power to the Shinigami through the Zanpakuto.
Shinigami have a tendency to try to fight at the lowest release level possible, and they have a better reason than they're pride to do it. Releasing their weapon costs them a portion of stamina. If the enemy didn't require that level of release, they just wasted energy that could have been spared for situations later. To compensate for this 'price of release', however, Zanpakuto abilities/techniques tend to be cheaper relative to the whole, so over an extended battle a Shinigami will typically use less stamina overall.
Where a Shinigami will tend to have more over-all options by being able to fight in any given release as needed, it's less versatile than the other races who's abilities are all available during every progressively higher state. Kido, however, more than makes up for this, granting Shinigami access to a broad range of techniques unrelated to their Zanpakuto.