A fresh issue increases the number of shares outstanding, which dilutes existing holders but strengthens the balance sheet.
An offer for sale provides an exit to founders or early investors and does not change the company's cash position.
An easy example
An issue of Rs 1,980 crore with Rs 900 crore fresh and Rs 1,080 crore offer for sale means most of the money does not reach the company.
Important note
Neither structure is automatically better, but it changes who benefits from your money.
Common mistake
Assuming the entire issue size funds the company's growth plans.