[scikit-learn] Can y of datasets be increasing/decreasing ratio when train regression model?

[lampahome]

I found many cases in kaggle to predict the quantity or trends. They all
set the real quantity as y.

But I have question is that does anyone set the changing ratio as y?

Like:

X     y
Day1  0.2
Day2  0.1
Day3  0.15
Day4  -0.1

y is the changing ratio compared with previous day.

Why anybody set the real quantity(ex: sales, car numbers...etc) as y rather
than changing ratio as y?

I want to know it's based on experience or other reasons

thx
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