A big advantage of using a SLR or DSLR camera is the ability to swap lenses to fit the job.

Almost as sure as a fate a DSLR-newcomer understands them he needs something else then standard 18-55 mm lens from a kit. Some wants “faster” lenses, some needs more sharpness, etc. 

It is not a secret, lens’ quality vary in different samples. Canon and Nikon (together with third-party manufacturers like Sigma or Tamron) are now making cheaper lenses available. This is good news because you can now buy these lenses at a lower price. The bad news is these cheaper lenses generally have a poorer build. They made from plastic to reduce their weight and price, but lost some part of quality. For most of amateurs such quality is enough good, but what to do when you want move up to [almost] professional grade?