Importance of correlation between sector and the broad market index?

I am currently conducting research on correlation between different sectors (TMT, industrials, etc) and the broad market (MSCI ACWI or S&P500). What’s the significance of their correlations in terms of risk management and portfolio management?

For example, if the correlation b/w TMT and S&P500 went from 0.9 to 0.6 during certain periods, what implication would it have?