Write to image/texture? Khronos spec says its possible to write to images/textures?

So while looking at working with image2D and image3D memory objects (which from my understanding are textures), there are write_imagef, write_imagei and write_imageui function calls (as stated in section 6.11.8.1, table 6.16). If images are texture memory which is read only, how can you write to them? Or am I not understanding something here? Thanks!

Images are passed as read_only or write_only parameters to the kernel.

In the former case loads go through the texture cache. In the latter case the texture cache is not involved.

Thanks. In the write only case, are there any performance improvements that using say, global?