We know that Yama, Vayu, Indra and the Ashwini Kumaras incarnated as the Pandavas.
As the sons of Kunti and Madri.

But why?
Why did they do this?
What was the need?

Is it just because Kunti and Madri invoked them and they obliged.
No.
There was a reason behind this.
Brahma had already instructed that you go and take birth on earth

This is after Bhumi Devi went and complained to Brahmadeva that evil has increased everywhere.
There were brutes and thugs everywhere.
A lot of people had become cruel and wicked.
Not just people.
Animals, birds- there was cruelty everywhere.
Veda and Vedic rituals lost their relevance.
Rulers became cruel and arrogant.

Why did Bhumi Devi say that she was unable to bear the load?
Not because of the weight.
Because of imbalance.

You must have seen headload workers carrying stuff on their heads.
If the load is not properly balanced, if one side is heavy, if it is tilting to one side, it will fall.
Even they might fall.

So Bhumi Devi was in trouble due to imbalance between dharma and adharma
Adharma will always be there, but then it has to maintain balance with dharma.

Here the situation was that adharma, evil had increased much much above the tolerable level.
The load is tilting to one side

So she needs help to restore balance.
That's why she went to Brahma.

Brahma instructed all the Devas, Gandharvas, Apsaras.
You all go and take birth on earth.
Not entirely.
Parts of them.
It is called Amshavatarana.
Only a part

When Indra incarnated as Arjuna it doesn't mean that he was not there at Swarga at that time.
Only a small part of Indra entered into the womb of Kunti and was born as Arjuna.

But then how did the load of the evil increase so much?

Hadn't Parashurama eliminated the entire Kshatriya Vamsha.
Again and again, twenty one times?
Even then the evil had increased.
That's why he did so.

One question comes up
If Parashurama had eliminated the entire Kshatriya Vamsha, then how come there are Kshatriyas now, at the time of Mahabharata.

Parashurama had killed only the males.
The Kshatriya women were still there.
They conceived from Brahmins and regenerated Kshatriya Vamsha.

For some time everything was good.
Then what happened; the Asuras were defeated by Devas.
Swarga had come fully under the command of Devas.
The Asuras started taking birth on earth, in hundreds of thousands.
Not just men, as birds, animals.
The purpose of taking birth on earth was to do good karma and attain heaven.

But their basic nature did not allow them to do any good karma.
Rather, they spread evil around.
They were cruel and harassed everyone.
They were greedy, intolerant, short tempered.
Many Asuras took birth in Kshatriya Vamshas and they became cruel and wicked rulers.

This was the situation.
They had to be eliminated.

But, who will lead them.
Devas are powerful.
But they don't know the ways and means of earth.
Someone has to make a plan and get it executed with the help of the Devas
They needed a leader.
So the Devas went to Vaikuntha.
Bhagavan said: Yes, I will come with you.
Let's get this done.
Let's relieve Bhumi Devi of her trouble.