Boon in investments can be any short-term gain on account of any unexpected change in the market scenario on a macro or microeconomic level. For example, any changes in the tax structure that allows taxpayers to save more taxes, lowering interest rates to reduce interest expense, write off debts, etc.
Boon can also be the growth trajectory available to businesses on account of favourable changes in government policies, changes in the business structure, or capital due to mergers, acquisitions, etc.
On an individual investor level, boon can be termed any excess profits shared by the company to its shareholders in the form of dividends, bonus shares, right shares, etc.