Warikoo-putra, lets talk when you've seen an equity downturn which wipes off 60% of your equity portfolio in 1 month.
Now imagine you have a goal coming up that needs a substantial outflow. Say your wedding.
What do you do other than pray that the equity markets go up? Would you sell the rest of your equity at a loss?
Fixed income instruments - such as FDs - are needed to park your money in when your goals move to the short term. That way when you have an impending outflow, the value of your investment doesn't have any volatility. Short Term = less than 3/5 years.
Early in career folks don't need FDs not because they are a bad instrument, but because the have no major goals to spend for other than the occasional vacation / iPhone.