so the question is: what is 3d texture and is there some real usage of it?
a 2d texture has 2 axises ( x and y) and a 3d texture has 3 axises ( x,y,z), you can think of it as many 2d slices.
and the usage… well to texture very complex objects with marble or similar, but that can be done ( even if it can be hard) with normal 2d. But preform realtime CSG and a 3d texture will help you alot. Then as often these days, use the texture as a huge lookup table to preform different math functions.
And to use MRI scan data and look at different layers, or yu can store small animations and get interpolation between the frames.
basicly, you get a new powerfull toy, and its up to you to find the use for it )
(did i mention that i really like them )