Shaders facilitate graphics rendering that is both flexible and efficient.
Once upon a time all graphics were done in software. This was very flexible, but not very efficient.
In the mid 90s add-on graphics cards became popular because they vastly accelerated graphics, but at the cost of flexibility. Writing pixels became a lot cheaper but reading them became much harder and/or slower.
Now that graphics are faster, and technology has improved (or become more affordable, depending on how you look at it), some flexibility can be added back into the graphics pipeline. Shaders give you this flexibility. The syntax is quite low-level because the hardware it is aimed at is also quite low-level. It's a compromise.
Really good rendering quality can indeed be achieved without them, depending on your definition of quality. You can stick to CPU-based graphics, if you don't mind those graphics being quite slow. Or, you can use the GPU and used the old fixed-function pipeline, which gives you a subset of the shader functionality. Whether that subset is "really good" or not is subjective - it was certainly good enough for many years - but you miss out on some interesting modern rendering techniques as a result, as the fixed function pipeline has no way of performing the completely arbitrary transformations on vertices and pixels that shaders can perform.
