Writable discs cannot be written at any speed, so you are forced to set a burning speed to avoid a coaster.
If you watch the disc box, you can see that discs are certified for a certain speed. Some discs (nowdays more difficult to find) were certified to be written only at 4x, or 8x max, so it is not always safe to write these discs at a higher speed.
Even a 16x certified disc, moreover, very often can be written better (i.e. with less errors at the end of burn) at a lower speed (12x or 8x).
Basically, each disc mediacode has an “optimal” burning speed, so it is necessary to select the proper speed when you burn a specific media.
Hard disks, on the other side, have a very different mechanism to write data on the platters, and this allow to read and write at a much higher speed compared to optical drives.
I hope that this makes sense