Going by the OP's definition...
1. Evolution. Good is altruistic. You take care of everyone. With sufficently advanced magic or technology, you bring everyone in a stasis. There is no evolutionary pressure as in a good society, everyone gets CLW and Remove Disease... a world with only Good would become an utopia in stasis, everyone would be fed Goodberries, no need to work, which will ultimately lead to the fall of the whole race if they can't adapt to their changing environment and rely only on magic, and magic itself can fail. So, maintaining societies that are evil, where "survival of the fittest" is a thing, can help have a population where the herd is effectively culled, creating a population from which civilization will rebuild when the magic inevitably fail at some point in the feature. The Neutral will oppose Good when Good is about to win, which means that Good must fight Evil, if not, Good would just indefinetely coexist with Evil, so by the OP's definition Good can't be just passively Good or defensively Good. Muscular Neutral will get involved in order for Good to just stay Good within their societies, and let Evil societies thrive to let the gene pool continue to evolve positively. In this scenario, your Muscular Neutrals could be worshippers of Gods of Nature.
2. Art. Muscular Neutral see that the suffering, accursed artists create the best art. In a Good society, they don't see as much valid art being created, while harsh environment creates the conditions for artists to emerge. Muscular Neutrals are taking the side of Evil because they love art when Good is about to win, helping Evil societies to thrive. But when Evil societies try to invade and destroy Good societies, Muscular Neutral will help them, because Good societies are much stabler and more apt to preseve teh created art. Basically, Muscular Neutral would be metaphisically inclined to put art above everything. In this scenario, your Muscular Neutrals could be worshippers of Gods of Art and Beauty.
3. Balance. Muscular Neutral are as convinced of the validity of their ethical position as are Good and Evil. Good is persuaded that Benevolence makes right, Evil is persuaded that Might make right, Neutral is persuaded that if left unchecked, they will fight. Muscular Neutral are the force that will side with the one who would be losing (including supporting the evil necromancer and his mummy lords army) just for the sake of preventing Good from deciding they can't be Good while standing idly when evil is being done, and they would be supporting the Good armies if the Evil ones are considering just pludering the Good countries. So, instead of a 1-vs-1 fight, it would be a 2-vs-1 fight, possibly with Muscular Neutral changing sides mid-war once an equilibrium is reached. After some time, Good and Evil coexist because of the risk a worldwide war would be for them. So, global war over morality can be avoided, which would be the imperative of Muscular Neutral ("we're the nukes that prevent others to initiate a war). Basically, their existence is the reason good is "altruism, respect for life, and concern for dignity" as per the OP and not "altruism and respect for life, so we must oppose civilizations that do not value altruism and practice slavery and ritual sacrifices" -- when Good try to be invasively Good, it is obliterated by the combined forces of Evil and Muscular Neutrals. In this scenario, your Muscular Neutrals could be worshipper of the God of Peace.
4. Harvest. Actually having an eternal struggle between forces of evil and forces of good is immensely pleasing. It allows people to feel elated, either by the hope of becoming heroes (on the Good side) or conqueror (on the Evil side). Neutral actually metaphisically enjoy the struggle between both sides and will ensure that no side gets to disappear, actually taking steps so they are of roughly equal powers so each side can get a lot of inspiration (if one is too reduced, the hope of becoming a hero is greatly reduced vs the risk of becoming the next in line to be ritually sacrificed by the evil side, and if paladins are all around keeping order, trying to conquer anything with your meek army of 3 zombies will fail long before you reach the "undead army overlord" level. Muscular Neutral side because they like witnessing this struggle, feel it's part of what makes "great souls" to emerge and they want to cultivate those great souls, so they actively ensure an equilibrium between Good and Evil. [EDIT: or if you prefer, fighting against all odds is what makes great souls, so Muscular Neutrals helps criminals in Good societies and prop the rebel underdogs in an Evil societies at the same time]. Neutral Gods gets much more sustenance from eating "great souls" when they die than commoner souls, after all. In this scenario, your Muscular Neutrals could be worshipper of the God of Afterlife Lunch.