Is it legal to RENT money? Why ?

Is it legal to RENT money? Why ?
Ist it possible to rent money?

This concept came across last week between beers and recreational creative thinking, and i still find interesting, recreational, for discussion.
We know about the idea of a credit, or about funds or assets.
Now listen:

Question open:
How can one rent money, and what would that mean?

Here is the most comon answer i got:

How to rent money?:
1. if you would like to rent money to a friend, you first set the amount of money to be rented, and the price of the rent.
2. If is money rented per day or per week or month, all varu according to your agreement
3. your friend must pay the rent on time, otherwise, extra fees may apply.

QUESTIONS

1. Is to rent money legal?
2. why would that be different from a standard credit? is it really a different financial product?
3. Why you would prefer to never ever rent money?
4. Or what do you mean to rent money is?