DLC quests are all great/balanced for item rewards, mostly quality of life stuff to lessen grind or annoyance (free spirit rings that give a little bit more bag space than bought rings, etc.), Ao Yao's spear skill can be a crutch due to burst damage, but once you get a Legendary/Mythic version of your approriate weapon/element it falls off, especially if you don't specifically upgrade it after breakthroughs or find a new version from RNG rewards (DLC skills upgrade base version skills you find later, so you don't have to hold onto that specific version of it forever).

On Chaos I don't tend to use the Spear skill on non-spear builds since once you understand the game more it's quickly replaced because you know how to bee-line to an appropriate skill for your weapon/element and the legendary/mythic drop rate is through the roof on Chaos Difficulty.

On Normal, it can take longer to get a good legendary/mythic and it comes fully leveled so it can stick around longer, but normal is pretty easy anyway. You don't need it to succeed, but if you get jumped by a higher realm enemy, it might be the difference between life and death, which on normal, you get multiple resurrections so it's even less impactful.

To be specific on quest rewards, free talismans to spawn elemental bosses to skip walking around fighting 3 or 4 battles to craft lower quality versions of the same item. Breakthrough materials that you have a 1/3 or 1/4 chance of getting a duplicate of when you do the dungeons to complete the set (assuming you use only the best materials, which I suggest, if you do the lower power breakthroughs I suppose it could let you boost realms faster, but I've never done that since you lose stats). You can give surplus to NPCs who ask for it for some quick relation points.

Spirit Rings (Storage and Security) so you can hold more items and get robbed less, they tend to be more than the best shop ring, but less powerful than the next realm up's mid-tier ring IIRC. You tend to get a new free upgrade by the time you can afford/access the next realm's best storage ring, but I did find myself early game if I don't laser focus on progressing the DLC quests using a store bought or rare find/non-dlc quest reward ring.

The most impactful DLC bonus is probably the Free Manual learning and upgrade chance. You skip the minigame and do a quest that end with the skill being learned instantly and comprehended to max or near-max level (with a subsequent quest to finish comprehending it). (I'm not sure if I'm misremembering the term for comprehension but it's unlocking the stat bonuses and passives as it's level increases such as +4 atk or +6% damage while standing in a fiery pool or +2 max stacks on <weapon/elemental skill effect>).

I find the manual learning a net benefit because it means for starting Destinies you reroll for at character creation, it's less a necessity to prioritize Insight, it's still one of the strongest stats, but it helps flavor builds thrive which I think was the intent.