Bhagavad Gita Verse 8, Chapter 4

paritraanaaya saadhoonaam vinaashaaya cha dushkritaam |
dharmasansthaapaanaarthaaya sambhavaami yuge yuge || 8 ||
 
For the protection of the good, and the destruction of the wicked, I manifest in every age to establish righteousness.
 
paritraanaaya : protection
saadhoonaam : good
vinaashaaya : destruction
cha : and
dushkritaam : wicked
dharmasansthaapaanaa : establishment of righteousness
arthaaya : for the purpose of
sambhavaami : I manifest
yuge yuge : in every age
 
Shri Krishna further explains why he manifests himself. He points out three reasons: protection of good people, destruction of wicked people, and establishment of dharma. This is not a one time occurence. It happens continuously : "Yuge yuge" means in every age, but it also means at every juncture, every crisis where there is a conflict that can disturb the harmony of the universe.
 
Now, from our limited perspective, we may not be able to appreciate why, when or how this restoration happens. It will usually never happen for the benefit of a single individual, city, state or country. It will happen for the sake of the entire creation. And just like sometimes a part of the body needs to be amputated, it could manifest as a tornado or earthquake, causing tremendous destruction. We could also complain that why does not Ishvara manifest himself and fix all the ills of the world that are happening now. The manifestation happens only at the right time, and only when it serves the purpose of the entire universe.
 
Sant Jnyaneshwar comments elaborately about Ishvaraa's avataara. He very poetically says that the avataara destroys the kaajal (kohl) of ignorance and lights the lamp of wisdom in the world. Also, he says that even though Shri Krishna commands Maaya in order to manifest himself, he is neither the doer nor the enjoyer of the actions that he performs.
 
Having manifested in the world, and restored the balance of dharma or harmony, what happens next? Ishvara very quietly becomes unmanifest and lets prakriti take over, just like clouds disappear after rain, or a fever disappears after we recover from an illness.