Prepayment and Prepayemet of income

Dear,
In text book, they said,
1)Prepayment reduce the expenses, increase the profit and asset.
Why? I thought prepayment only transfer cash to a new form of asset being called prepayment and leave nothing change in the current fiscal year.

Assumption I have paid 10$ to gas acc for this year but in fact, the actual expense in this year only 8$, prepayment = 2$

CR Cash or bank acc 10$
Dr Prepayment 2$
Dr expense 10$ (in gas acc)
Cr expense 2$ (in gas acc)
(net 8 will be charged to I/S and reverse 2$ to Dr in Gas expense acc)
I see nothing change except 2$cash become a new 2$prepayment asset

Why are the entries in text book like:
Dr Prepayment
Cr Expense.
It make the whole picture look obscure.

2) they said,Prepayment of income reduces income, profit and increase liabilities.
in mine,
CR defer revenue (liability)
DR cash received or bank acc
i see only cash increases and liability increases . Why do income and profit reduce? Please explaine for me?