What is a transformative battle?  This is a battle wherein a nominally average or even sub average character is suddenly transformed into a completely different power level, changing the entire battlefield and wreaking havoc on the other guys plan.  It basically defines an "Oh, s&$*" moment.  

These are for sure a nerd trope and common in many films and comic book.  The most obvious one is of course the Hulk.  The number of times mild mannered Dr. Bruce Banner gets pushed around by bullies or the local biker gang only to go green and tear ass through the local color is astronomical.  This is a special kind of transformative fight as Banner really has no control over the Hulk and in truth would rather not have him go berserk. 

However there are many others.  In the great movie the Iron Giant the lovable giant robot who befriends a local kid and plays hide and seek in the junk yard sees his friend killed and transforms into an unstoppable killing machine, much to the chagrin of the 1960 US Military.  

It's not only the good guys who do it.  In the movie Mortal Kombat Lui Kang grabs the weird little lizard dude that has been shadowing him the whole movie and all of a sudden it transforms into Reptile for the most epic fight of the film (I saw this movie in a theater in 1995 with about 400 other teenage guys and when the real Reptile finally came out it was the closest I've been to a massive riot and I've been in a real riot).  

It can catch you off guard at times too.  In the Samurai Jack episode Jack and the Annoying Creature Jack is pestered by a cute but annoying creature who looks like a Studio Ghibli reject and tags along to more or less falls in love with him.  Later on when Jack is getting destroyed by 50's Greaser robot bounty hunters the cute cuddly creature turns into a gigantic demon and wastes them all in short order.  

That is the true definition of a transformative battle: one where you kind of expect things to go one way and all of a sudden all your expectations are tuned sideways and the entire battle becomes a whole new one.

So why do we love them?  There are several reasons.  First off everyone loves to see the underdog get the upper hand.  When the wimpy kid who gets pushed around his whole life suddenly gains the power to strike back we all feel a sense of justice about it regardless of how good or bad that kid ends up being (a la Chronicle).  

We also love seeing clever tactics upsetting a plan based on pre-consieved notions.  In the original Transformers TV show Megatron thought he truly had the upper hand when he ordered the Stunticons to merge and form Menasor but Optimus Prime turned the tables when the Aerialbots formed Superion (God I'm a nerd).  

And finally we all love surprises and twists.  In Game of Thrones it totally looked like the Viper had taken down and was going to kill the Mountain but he made the mistake of getting too close and got pummeled to death when it turned out the Mountain was less dead than it appeared.  

So relish the idea of the beaten wimp turning into a combat monster.  I doubt Hollywood will drop that idea any time soon (or the comic world) and try to keep that in mind when you are dealing with someone with considerably less power than you.  You never know when they might get bitten by a radioactive spider.