Symbolic goal
Near the beginning of the trailer the player ascends out of the water. When he does a very distinct “objective” is revealed:
This seems like intented and great symbolism: it is a way of telling the audience that reaching for heights is an important part of and goal in the game.
Later in the video you fly past this objective so you can see that there is a fire on a tower there. It begs the question: is it the goal to light these fires in high (and hard to reach) places? What is their purpose? Why do you go to them?
Interesting places are high up
The most interesting places are high up and are not easily reachable. The following places seem not accessible by basic climbing.
Tower of fire
First there is the tower on the mountain:
While a mountain may be climbable by hand to some degree you probably need some kind of climbing equipment to reach this tower.
It is quite likely the shown glider will go hand-in-hand with special climbing gear which allows you to climb more efficiently. Just like in The Legend of Zelda.
Side note: the tower appears to be more than ten meters tall itself (the railing at the top indicates this) and it is contructed in a non-modular way (in contrast to the manually constructed houses that do show modularity) so this stucture is unlikely to be player made. Here is an explanation of their most likely purpose.
Worn Sphere
This “worn” sphere is quite distant and therefore quite high. The only visible entrance is also high up. Since you probably can’t climb up a sphere towards that entrance you need other means to reach it.
Floating Cube
The floating cube is completely unreachable by climbing (judging by its shadow it is probably several tens of meters large and therefore even higher in the sky.) It is only reachable by levitation or mounted flight.
Floating Island
The floating island is even higher in the sky. Probably hundreds of meters. It likely requres a dragon to reach.
When you zoom in you can get a much better look:
The floating island contains large buildings that are much larger than the trees (and doorways) on it. There seems to be a landing platform for flying mounts/dragons on it (to the right.) Since its light house emits green light it likely associated with the green magic (of movement.) It is a very interesting place.
Maze Sphere
The “maze” sphere seems very far and very big (and is shown at the end of the trailer.) It is possibly the highest structure seen. If it too can only be entered from high up, it most likely requires a top tier (high flying) dragon to get there.
Height matters
While having the high ground has some natural advantages in any game (like the ability to see farther) in Light No Fire it appears to have special meaning. The most interesting areas in the game are only accessible when you can reach a certain height. It is likely the primary way to divide the game into different tiers.