It makes sense if you’re carrying on with dollar cost averaging as you have in the past. Automatic witholding/deposits from your regular paycheck are a good example. Market went down? Cool, I get more shares for my dollar over the next few months/years, and I have a reasonable expectation that the market will recover in the longer term.
As @Octopus said, human psychology is a major part of stock price fluctuations. Benjamin Graham said it:
“In the short term, the stock market is a voting machine. In the long term, it’s a weighing machine.”
Investors tend to wet their pants when there’s a real market downturn, and they sell and make the downturn in share prices even worse than objective facts suggest they should be. People come back to their senses after a bit of time and start paying attention to market fundamentals, and then prices recover to the point that they reasonably match with market reality. So yes, buying when the market is down is a good thing. The challenge, of course, is knowing whether the market is down already, or still has a long way to go before it’s truly hit bottom.
For the average personal investor has no idea what the market is going to do next (which is most of us), dollar cost averaging and annual portfolio rebalancing together are a solid strategy for managing investment risk. Decide on a target stock/bond ratio for your whole portfolio, and keep salting money in from your paycheck on a regular basis. Once a year, rebalance your portfolio: if the stock percentage is below target, sell some bonds to buy stocks, and if the bond percentage is below target, sell some stocks to buy bonds. This isn’t market timing, since it happens at the same time every year and you don’t care whether the market as a whole is up or down.