and your 2nd question:
The main reason of tapering the wing is to reduce wing tip vortices and therefore reduce induced drag.
Your question is kinda backwards to expected, advantages of a low taper ratio (i.e more rectangular)
The optimum planform for reduced drag is an elliptical shape, and at the opposite end is a rectangular planform.
However, reasons for lower taper ratio include favourable stall characteristics (most important) and lower cost.
A compromise is found by tapering the wing.