Whether you want to use all your funds to buy at once or divide it and start to buy in bits depends on your strategy and if you are a trader or investor. Buy in bits if you are aiming for short term profit or you can decide to buy in lump sums if you are investing for a long term. Cryptocurrencies are volatile and it is not advisable for traders to use all their funds to place trade at once so if they lose they will have reserve funds to trade again. But if you're a crypto investor specifically Bitcoin you can buy anytime when you have the funds or you can do DCA method because Bitcoin has the potentials to always reach ATH and give you profit.
The strategy described by Op is mostly beneficial to traders who are likely to lose their money in very short term. Due to high risk involved in trading, it's advisable for traders to use a portion of their money to trade and reserve the rest for later incase they lose their initial deposit. As for Bitcoin holder who plan to invest for a long time, it's not necessary to divide your money into segments as far as its the amount you can afford to lose, you can invest it and follow up with subsequent accumulation to continue growing your portfolio. Regardless of whichever you're doing between trading and holding, bear it in mind that you're taking risk hence use what you can afford to lose either way.