Example: $600 a month for four months at prices of $30, $20, $24 and $25 buys 20, 30, 25 and 24 units, or 99 units for $2,400. The average price paid is $24.24, below the $24.75 simple average of the four prices, because more units were bought at the cheaper levels.
The real benefit is behavioural. A schedule removes the decision of when to buy, which is the decision most people get wrong. It also matches how most people actually receive money, in monthly instalments from income.
Understand what it does not do. It does not reduce risk over the long run and, for a sum already in cash, it has historically lagged lump-sum-investing more often than not, because markets rise more often than they fall. It is regret insurance with a cost.
Related: lump-sum-investing, value-averaging, asset-allocation, sequence-of-returns-risk, rebalancing