Percentage changes are not reason to invest. They might be reason to take a bet, but to invest, you are considering what is causing those changes. You might be wrong, but you are at least considering some sort of evolutionary process.
Investing on a percent change is an economical school based on the assumption that ALL the information is already in the market.
And you might never find that information elsewhere.
Let's take the senator who sold $1,6 millions worth of stock based on his covid knowledge:
That info was privy to him. The fact that he sold airline's stocks is probably available in the %age change of the day. Probably airlines under performed the S&P that day. So without knowing what's up, only by noting the discrepancy in market movement, you might deduce it's time to sell.
So investing only according to how the market move, is economically sound in my book.
But.. I was short in the middle the 1st fall in the end of February. Then lost money when the market rushed back up.
And according to the "Information is in the market' theory, I closed my short, and when leveraged long ... and lost my 50% gain from 2019 in one week of march. So that 'market contain all the information' might not work so well