First the game does not need to present all of them to a new player
but they should for the experienced old timer?
ofc, UI should tell you at a glance all you (really) need to know .... but those are very general things.... is it a rural or a city building, is it a cavalry unit or a peasant levies, is it an IO or an Empire with 2 personal unions.....
while learning and understanding the machines are an very important part of any gameplay experience, double so if you have no prior experience and clear and useful UI makes this process sooo much smoother.... That not the main point
the usefulness of IO (and let's put aside that the term feels a bit too contemporary ) is not that it's telling it to you ''
they are not like anything else''.... but it's signifying that it's not a simple diplomatic/dynastic agreement you can break or call upon, it's not the economy nor the society you can tweak here or there and not even internal makeup of your sovereign tag..... it's telling you it's ..... ''something else''

it's telling you it's binding to the other entities in more than just a one-way street with another, it's a culture of obligation you can't just leave or enter into. It doesn't matter if your ruling dynasty changes or you convert to the heretic religion or if you change your mind and wanna try something else. It's a system you can't escape regardless of your wishes and interests. And it doesn't matter if HRE is an orangutan and an Ilkanate the orange nor the shogunate similarities to the sandy beach. It's a system of order. And the persistence and robustness of it, goes beyond of an .... hey, I have an idea, let's create a commonwealth just because it will allow us more efficient administrative management.... it doesn't matter if one system is based upon legalistic interpretation of religion, the other on economic ties to each other and the other upon the values of family & conquest.
You are here, and you are gonna play this game. Or try very hard to break the wheel and set up a new order.
If you wish, it's an ideology
edit: i can't help myself.... omg i can't discern because they are all labelled an IO, what's the difference between the Shogunate, the Banking Syndicate and the One Roman Empire..... omg an orange doesn't translate into what an orangutan tastes like nor what is the sound of the music.... hmmm, is it in the Prince where such people are categorized as serviles and unfit to rule?