"It's a very very... dead world..."

This is an issue that has largely plagued open world games and, while it has gotten better, still kinda does. That the game worlds feel somewhat devoid of life.

In the real world you often have many small animals around you, birds flying about and chirping, insects buzzing, rusting in the grass and brushes and the like.
In many games however those are absent or relegated to predetermined events. Due to this, the game world feels a lot more sterile than it should be.

A small test you can make is turning off the games music, if it suddenly feels to silent and lifeless you know that there isn't enough ambient noise and things happening to make it work without it.

Adding animals would improve this a lot. Having flies and other insects buzz about, crickets jumping when you walk through higher grass, seeing small lizards and squirrels scurry about, spiders crawl over dungeon walls, birds flying about... All these would greatly enhance the atmosphere.
And similarly this would reinforce the opposite. Suddenly coming into an area that is dead silent and devoid of life adds to a great creep factor.

Now the main issue often brought up with it is resources and development time. Adding small animals take processing power, modeling them and adding sounds takes time to develop.
However for open world games this should have a much higher priority as the game world feeling alive and interactive. Only developing the mere minimum needed to make the world work has the effect of leaving it somewhat dead.

Another issue that is mentioned is “what would you do with those animals”. Here's the thing, you wouldn't necessarily do anything with them aside ones large enough to actually interact with. They would largely be there for the atmosphere. Nor would they all simply attack you.
Not every creature in the world has to be there to be used or to attack you. They could still react aggressive when cornered or be killed for a few drops but otherwise they would simply be “there”. After all, nobody would complain about the grass simply being there, even though that could even be used for some purposes.

As a few side notes.

• Small animals would do with relatively low poly models from a distance and only change to more high definition details when up close.
• Larger bugs can be a very simple model and small ones like flies simply sprites.
• Most small animals would simply flee unless you lure them so their AI would not need to be too great.
• Some small creatures would attack such as snakes, scorpion and larger spiders making high grass more dangerous but would be prevented by wearing boots.-They would not need preset spawn spots but simply an area defined spawn rate and areas where they can spawn (grass, forest, water...). The game doesn't need to track them so they can simply be despawned and respawned when an area is changed.
• Some small insects could become a danger when swarmed such as bees and ants, for that a swarm could simply be a lingering area effect.