Three purposes, stated without ambiguity. Krishna incarnates to protect those who walk the path of righteousness, to bring down those who spread injustice, and to firmly reestablish dharma. Not once. Not twice. Yuge yuge — in every age, as many times as needed.
Think of a farmer who returns to his field season after season. Weeds grow, he uproots them. The fence breaks, he mends it. The crop needs water, he channels the river. The Lord tends to creation in much the same way — not once and done, but with ongoing care.
The closing words — yuge yuge — carry enormous reassurance. No matter how dark a period becomes, the promise stands. The Lord will come again. This is not a distant hope. It is a structural guarantee woven into the nature of the universe.