There are several benefits that you can get by investing in the top 100 companies by market capitalization like:
Diversification – Invest in multiple stocks at the same time to minimize your overall risks and simultaneously diversify your portfolio across different sectors.
Stability – Most of these companies are well established and considered to be relatively safe stocks. These companies have well analysed track records thus are less volatile than mid or small cap stocks.
Best returns - Choosing a right stock at the right time can significantly boost your portfolio returns.