Mặc dù mảng lưu giữ thứ tự, nhưng nhiều khi ta không quan tâm đến thứ tự đó cho lắm

Along with all the above technical differences, I think there is a fundamental difference in the purpose and meaning of an Object and an Array.

The properties of an object DESCRIBE/DEFINE the object whereas
The elements of an array do NOT DESCRIBE/DEFINE the array, on the contrary the array defines what it’s contents are. Do note - I am not talking about technical aspects. You can have any combinations technically but semantically each has its own purpose.

For example a card holder. Each card does NOT DESCRIBE/DEFINE the card-holder. But the card holder does define it’s purpose - that it holds only cards.
An Object is used to represent an entity and its properties DESCRIBE/DEFINE the entity. Take the same example of a Card. A card has properties like color, number which DESCRIBE/DEFINE what the card is.

For your above example:

Each object which represents a person is defined by the properties id, firstName and lastName.
A list of these persons cannot be an object of objects because each id does not describe the object of objects. So

"users":[
    {
        "id":"id",
        "key2":"value2",
        "key3":"value3"
    },
    {
        "id":"id",
        "key2":"value2",
        "key3":"value3"
    }
]

is a better representation than

"users": {
    "id1": {
        "id": "id1",
        "firstname": "firstname1",
        "lastname": "lastname1"
    },
    "id2": {
        "id": "id2",
        "firstaame": "firstname2",
        "lastname": "lastname2"
    }
}

even though technically you can use either. I hope I was able to convey(put into words) my thinking in the right manner.
Nguồn:: Stack Overflow, Array of objects vs Object of Objects