Buying deep ITM options is akin to buying the underlying stocks/equity and the advantage is you spend less than you would buying the underlying. e.g. for GLD currently at $158, if I buy 100 shares of it, that would cost me $15,800.00 (less commission). But if I were to buy deep ITM options like a 150call contract would only cost me $995 for equivalent 100shares for GLD. The delta for 150 call is 0.96 and that means that for every $1 move of GLD, the options will gain $0.96. This strategy is when your prognosis is bullish for GLD. As for Puts, the prognosis is the opposite i.e bearish, and the strategy is to buy Puts that is deep ITM. Buying above market level (GLD at $158) means buy Puts at 170 strike with delta of -0.96 (deep ITM). So when GLD moves $1 down, the put will gain $0.96 because of that. (opposite to deep ITM Calls)
One of the key take away of buying deep ITM options is that it cost you less than buying the underlying itself but you enjoy the possible gain when the underlying moves in your direction.