The one time buying will not be comfortable with many people as they mightn't be ready for that type of risk but DCA gives you control to invest the amount that you want and when you want to.
It's not about your capital is small or big with that one-time purchase, it's about risk and possible profit you can get from that entry. With DCA strategy, no more importance of a perfect entry, because your entry price will be averaged with many entries at different time and prices. With this strategy, there will be no pressure to break your bank account and use all of its balance for a Bitcoin purchase. Less risk of entry with bad price, and less risk of empty hands for emergency after your Bitcoin purchase.