std::any is pretty cool. You can use it, for example, to build a map where the key is just the type of the value.

So, you could query it like this:

let maybe_position = store.find::<Position>(id);

The id is the ID of an entity which may or may not have a Position associated with it.

This is similar to just using structs/OOP, so where you'd have a Vec<Entity> and then you'd call entity.position, but the big difference lies in flexibility. An Entity type would need to have all fields defined, which may ever exist on an entity.
With this type-as-key map approach, you can just tack on new attributes to entities and dynamically react to them.

All of this is basically how the storage works in the Entity-Component-System architecture (ECS), which is popular in gamedev, for example. But both the storage method and the ECS architecture are good tools to be aware of in normal software design, too.