It's just a way to better cope with investing in bear markets: if you continue investing with the goal of reaching the projected 800,000 $ in 10 years, but you are in a bear market, you will have the impression that the goal is going further away, if the total worth is shrinking of 3% every month. You invest, and you stash instead is lower and lower! And you start (maybe) panicking
If instead you start thinking about number of shares, and thus your goal is to have in 10 years 4500 shares at the projected 10y away price of 180$, then during bear markets you are actually crying of joy because you want to buy all of them and the price is getting lower, so the goal of 4500 shares is coming nearer and nearer and faster!
Both are identical situation (projected 800,00$ in 10 years VS 4500 shares @ projected 180$/shares), only the perspective changes, but I found it very powerful.
And during bull markets you can always switch back to total stocks worth and you see the actual money increasing, so in the end you are psicologically always happy :D