Leaving modifiers in place is a technique that allows for a high degree of flexibility that is particularly desirable for something that stands a good chance of being customized. If someone else wanted their own SD card holder ring, it would be trivial to resize the ring and adjust to position of the SD Holder.
Also in this chapter, Blender's tools for precise placement were employed to make an object match real-life measurements. Blender may never be able to compete with other CAD programs for precision, nevertheless with some careful planning and clever manipulation, Blender is capable of extremely precise modeling.
The next chapter is a huge challenge where the tools and techniques learned so far will be applied to making a modular multi-part toy with moving parts. The modeling will be simple, but the result will be impressive as the ways that parts can be joined together and designing those joints to be printable will be explored.