The difference between steem leasing and delegating is same as borrowing money from bank and depositing it for interest. When you lease the steem from a portal, that portal acts like a bank which generates income from services it renders to the lessor and delegator.
If you have surplus steem power which is like a fixed deposit, which you donot wish to liquidate to steem, then you delegate to a bot who pays interest to you. When you need steem power you go to leasing market and borrow it for a month two or three then you are required to pay interest.
To conclude
Delegation = Deposit
Leasing = Loan/borrowing