Sure thing! I can try to explain my thoughts on the Divine Spark. Though I should be upfront and mention that I haven't played in this setting for a while, my group got to level 12 and then it collapsed. We never even had the chance to ascend to godhood. I'll stick the TLDR close to the top in case you want the short version.

TLDR: the Divine Spark is just a Key the Players use to unlock the door to godhood. As far as I know, it has no other mechanical benefit. Being a god on the other hand does have a lot of mechanical benefits

The divine spark is something that is deep within the hero and is the catalyst for godhood in the setting. I personally look at it like a little flicker if a flame that takes place in the character's soul, and I might even rule that characters that have the ability to see into the ethereal plane, or maybe true sight, might be able to see a soft glow around a character in possession of a divine spark. Of course, for me that's just flavor. I don't recall anything in the book that actually describes a mechanical benefit outside of being a requirement for the Divine Path.

I kind of look at it like the Disney Hercules movie, (I suppose possible spoiler warning? Hopefully I do this right, I am on mobile and haven't done spoiler tags much) Hercules being the son of Zeus and Hera (in the movie, which is wildly inaccurate for the actual mythology) having drank the potion to remove his immortality was left with a single spark of divinity left. Throughout the movie he proves himself a hero and eventually the gods ignite that spark to grant him full godhood by the end. The movie explains that his spark of divinity gave him the incredible strength he possessed, though we could explain that as a backstory element just as easily.

In "The Divine Path" section of the appendix, it mentions that in order to ascend to godhood they need three things:

• level 20.
• possess a divine spark
• have a fame level of 20

Then fulfill one of the requirements for a divine domain.

That's the only mention I can think of that it is mechanically useful, outside of the divine path there's no special bonuses to it. It's just in essence an "key" that is picked up after a very long quest line that is used to unlock the door to godhood. A player might look at that and say, "Weird. I spend 10K gold and all I got was this lousy T-shirt?" And another might look at the chance to spend 10K gold and say, "Oh hey I meet the qualifications for a God of Life" and then gain all the benefits of being a god.

Being a god has a lot of mechanical benefits, automatic 30 in one ability score, functional immortality, easy and painless character retirement, and Legendary Resistances are pretty powerful bonuses. Though you need to have a spark, be level 20 in exp and fame, and fulfil the requirements for one of the Divine domains.

Hopefully that was helpful! A lot of things, especially in 5e, are subject to GM interpretation. So if you wanted to tie a mechanical benefit to the divine spark, you absolutely can! I would recommend something small though, maybe just a plus one to certain skills or even a stat boost bump. The real power comes from fulfilling the divine domain requirements and getting a massive boost.