Once we have our data structure of choice in place, we might still want (or outright need) to have a lot of voxels loaded at once, which leads us to two issues:
It might seem like we have a lot of RAM (gigabytes, in fact!), but voxel volume data is big (it scales by the power of three:
n³) and our users may have other things running, like a web browser, so we can't just eat all of it!
The bandwidth between working memory (RAM) and our processor (CPU) is limited and shared with other processes, thus being highly contended!
To remedy these issues, we'll have to compress our voxels, for which there are many methods...
Note: Compressing voxel data is, of course, also greatly useful for persistence and networking.