There are much more learned people in this forum, but I'd just like to share my observations with regards to this.
1) All these stories where Asuras use their boons to create havoc, were written (I prefer compiled) to teach the humanity that power, when given to the wrong person always brings destruction.
It tells us that a bad heart, no matter how hard it tries or how close it gets, will never achieve immortality.
These stories show, on more modern terms, the difference between 99% and 100%. It tells us that a 99% complete task can always be reversed but a100% cannot. Just like near-immortal asuras can be destroyed but the immortal devtas cannot.
2) Vishnu or Adi Shakti are considered the leaders of the clans of the devtas. So, to defeat the Kings of the Asuras, the leaders of the Devtas are deployed.
Also, thinking of it as an actual human-like war, we see that to destroy a particularly strong opponent, (about whose strengths almost nothing is known) a side has to unleash the most fierce weapons it consists. Among the Gods, the most powerful are Shri Vishnu and Maa Adi Shakti. So, you have it there.
You'd ask, why not Shiva? He is the destroyer, a born warrior!
Well, Shiva can pacify Kali. Who will pacify Shiva?
If he's is enraged, he'd probably end up destroying everything (including the devtas) even before you can say "Shiva"!
However, there are many instances where the Devtas themselves get rid of the asuras (with ample help, yeah), one of the more famous instances being Indra's killing of Vrtrasur.
3) And, Brahma is the creator, the beginning. Vishnu is the conserver, the middle point, while Shiva is the destroyer.
So, Brahma's comfort zone is creating, which he does. Shiva's comfort zone is destroying, which again, he does at the right time.
Between them, Vishnu is the multitasker. 😳
And so, Brahma does not take part in any violence.
However, without him, in 99.9% cases, the devas don't know what to do, which Shakti to approach etc. So Brahma does play a key role in their destruction.
Hope this was helpful. 😳
Feel free to ask in case of further doubts, I'll only be too glad to help out as far as I can.
Vasudha
Mannat Har Khushi Paane Ki
