Ask whether a Systematic Investment Plan (SIP) beats investing a lump sum, and you'll get confident answers in both directions. The honest answer is that it depends — on the market's path, on your behaviour, and on what you are optimising for.
Purely on average returns, a lump sum invested at the start tends to come out ahead over long periods, for a simple reason: markets rise more often than they fall, so money invested earlier spends more time compounding. If you had a large sum and perfect discipline, the mathematics mildly favours investing it at once.
But that ignores two things that matter enormously in the real world. The first is sequence-of-returns risk: a lump sum invested just before a sharp fall can sit underwater for a long time, and few investors have the temperament to hold through that. The second is behaviour: an SIP automates investing, removes the temptation to time the market, and turns volatility from a threat into an opportunity, because you buy more units when prices are low. This is rupee-cost averaging.
There is also the practical reality that most people don't have a lump sum — they have a monthly salary. For them the SIP-versus-lumpsum debate is academic; the SIP is simply how their savings accumulate, and that is a perfectly sound way to build wealth.
A reasonable synthesis: if you have money already sitting idle and a long horizon, staggering it over several months can reduce regret without giving up much return. If you are investing from monthly income, the SIP is the natural and disciplined choice. Either way, the biggest driver of your outcome is not the technique but whether you keep going through the inevitable rough patches.