Seems there are still a lot of people that didn't get how SSD will be a game changer
I found this gif in resetera, guess this will make things easier to understand
Now this is from HZD, but this could also apply to any kind of game, like even fighting games (specially 3D)
Imagine you're playing a character in a 500m2 field, with all kind of stuffs (objects, animals, houses, castles ect)
With HDD this tech is simply not possible with high end graphics, so in that case usually all the 500m2 are actually display in the game, even those you don't see.
As you know, the little stuff GPU has to display, the more high of visual quality you get, for example in an open world, let's say RDR2, Artur could be much much more detail if he's the only thing that is in the console and that the GPU has to display, but of course it won't be a real game in that case lol, so there has to be a world around him, and so the more you add around the character and the world, the more it's hard to maintain high quality graphics, this is why games like RDR2 or GOW are praised, some other games are beautiful but they don't have the same level of detail and life in their world. That being said:
Now with SSD, especially one that is super fast, the console will only have to display the part you're seeing in the screen like shown in the gif above, so like only 25-33% of that 500m2 need to be display by the GPU, thanks for the SSD super fast streaming capacity, no extra work is needed for extra textures, objects and such that aren't in the screen anyway, so the GPU won't have to work near as close as much as if we were in the classic HDD model, that's why, or at least one of the reason devs are mostly excited about PS5 specs, and says you'll need to see the whole specs and not just the TF part.
Tried to be as clear as i can, hope i did lol