and DCA helps if market is not trending, ie mean reverting

 

if market is trending UP, you want to go all in at the very beginning.

if market is tredning down, you want to wait until the every end to catch knife.

the problem is you only know which way market is trending ex post.

SPY tends to mean revert and good candidate for DCA.

QQQ tends to trend more, but it can trend in both directions. It is not a good candidate for DCA.