FIIs have deep pockets and thus have access to extensive financial infrastructure as well as the best talent. They make financial decisions with the objective of maximising their risk-adjusted returns.
FIIs are extremely market savvy and keep track of vast amounts of real time data to help them make the best decisions.
However, this does not mean you should blindly copy their buy/sell decisions. Their specific investment objective and time horizon are not parts of publicly declared information about their investments.
Also, FIIs are infamous for jumping in and out of investments very fast. Thus, its easy to confuse their trading with investing.