We don't know they're unnecessary yet, it said place all 4 on a map and Raftel will rise, that may be literal and the 4 poneglyphs may have to be placed at specific locations to bring Raftel to sight, it's not that uncommon in treasure stories.
That is a possibility. There are indeed many movies, books and the like where people have to find a certain key or lever or several parts of a key to open something. However the situation here is quite different. The key parts are four huge square stones that have to be placed on four locations that are separated from each other by huge distances. If they were close to each other, finding one poneglyph would be enough as you could just cruise around to find the rest. The only way that this system of four locations works, is if they are so far apart you can't possibly find the other three locations if you only know one. That's the point of this system: forcing people to find all four parts.
Of course when they are that far apart, how could the four poneglyph trigger anything? That would mean there is some kind of a massive 800 year old world-wide mechanism that connects all four locations with each other and that only gets triggered by four specific huge square boulders and not by any kind of replacement that has the same size and/or weight. Even for the OPverse I find that over the top as you would have to visit all those locations.
So personally I don't really favor this possibility as I seriously doubt these four locations were created intentionally. I think it just happens so that Raftel is located exactly on the crosspoint of four other islands if you would connect them with each other and they used that as a means to find Raftel. If only the contents of those four poneglyph matter, you can use their information to create a map. Then you need to be able to insert that map into an overall map of the New World and if you succeed at that, you know where Raftel is and to achieve this result, you don't have to move anymore. You can assemble the map as long as you have the info. This system is also much more safe as you are forced to find all the info of the four poneglyph, you can't really rig it. That's different if there was such a huge mechanism.
Ahh okay.. I see what you mean... that does make sense.
My main reasoning though, was because imo, I thought it was better to use Observation Haki to actually see people (the white aura thing.. the same way that Fujitora "sees" people) rather than just hear people..
IMO I thought Seeing > Hearing when it came to Observation Haki..
They pretty much come down to the exact same thing. Oda worked around the concept of Haki in quite a weird way. Post-time skip everyone who uses Armaments, gets a black color, but that never happened pre-time skip. Despite this the results were exactly the same in both pre as post-time skip. I never understood why exactly Oda made what seems to be a huge contradiction as I can't imagine he did it by accident. So I started to think he wanted us to perceive the concept of Haki pre-time skip the same way how the Strawhats did: they didn't or barely knew what it was and Oda visually represented that and he did that the same way with the Observation Haki. He only starting clearly showing people as white aura post-time skip, but it already happened pre-time skip. Hearing and seeing are two sides of the same coin, you can't hear someone without having it being tied to a location. Otherwise it doesn't make sense. This was portrayed quite clearly during the Skypiea arc and also during Marineford: Cobi and the girl felt people voices disappear all around them. It's rather useless to hear voices if you don't know where they come from.
The two can't be seen independently, so there's really no such thing as seeing > hearing or seeing < hearing. It's likely however that the further away someone is going from you, you probably first will lose the ability to hear him before the ability to see. Being able to hear and see people over long distances, is highly OP. That's what Enel did by amplifying his haki with his DF and as a result he could hear and see everyone on Skypeia and bring immediate punishment on anyone who even talked bad about him. Simply the act of seeing someone from a long distance has been portrayed regularly.
