Dollar cost averaging used to be promoted by mutual fund salesmen (sometimes called financial advisers) as a means of getting a steady flow of commissions from  investors.

Here's how they used to explain it:

You invest the same $ value each purchase. Say the first purchase is 200 shares at $50, then the price goes to $100, you buy 100 shares. The average share price was (100+50)/2=75. Your average cost is 20,000/300=$66.67. You have gained $8.33/share.

 It is just a trick of mathematics, using the difference between average and weighted average to confuse the uneducated investor. It is nonsense.

Putting a fixed percentage of your income into investments every month is a great way to build up savings. However, if you have the money to invest, then investing it now rather than spreading it over a long time period will come out ahead most of the time.