Duration and parallel shift in the yield curve

I don’t understand why the parallel shift assumption exists in the first place. I even checked my level 1 notes and I am still clueless. Pls help!

It’s the easiest way of translating the impact of duration to bond price. And its inline with how we interpet duration (%change on bond price given a change in yields)

Example, if the modified duration of a bond is 5 and there was 100 bps parallel shift upwards in YTM, then bond price is expected to decline by 5%.

We use a parallel shift so that if we have a portfolio of bonds of various maturities, the yield change is the same for all such bonds.

For a single bond, the shift doesn’t have to be parallel; all that matters is the change in the par rate at that bond’s maturity; i.e., the change in that bond’s YTM.