Consistent, repeatable parallel alignment of accessories within a drawtube or focuser is important for imaging and for collimating, and it has become even more critical with the popularity of ultra-fast focal ratios, larger image chips, and precision collimating tools.
There is a misconception that the problem of the cylindrical slip-fit system we use for eyepieces and other optical accessories is centering. There are adapters and holders on the market that are advertised as "centering" or "self-centering", but even if an accessory is pushed off-center by a millimeter it will not introduce a significant problem if the accessory and holder axis remain parallel. In contrast, angular misalignment between the accessory and the drawtube axis of even a few arc minutes can be detrimental, especially for collimation and imaging. The bottom line is that centering is nice, but parallelism is golden.