Do you get paid your salary a whole year in advance? No.
Thus this distinction is important. As noted in the wikipedia article above the rationale for this has to do with "I have a big load of cash right now, do I just put all of it to work now or slowly over time?"
Let's say my career potential earnings are $1000.00US. Why can I not consider that my bucket of cash (even though I do not have it on my person all at once)?
By depositing 26 times per year, am I not putting my money to work "slowly over time"?
Scenario 1 [S1]: I have $120,000 in cash.
Scenario 2 [S2]: I expect to make $120,000 in cash over 12 months, equally once a month
On Day 0...
S1 - I have two choices: put all $120k in the market ("lump sum") or DCA is over 12 months.
S2 - I have no choice but to DCA it.
If I choose to DCA in S1, then I could have made more money by lump summing (depending on market conditions). This is basic TVM.