we are all from science/engineering background. So let me put it this way.
Algorithmic trading is an engineering product, which bring buyer and seller together. Essentially, there is nothing different between automated trading systems and eBay or Amazon.
Algo trading is not about providing liquidity. There are plenty of liq-taking-only strategies running in the market, and they seem to be still profitable.
Algorithmic trading's contribution is---just like the contribution of eBay and Amazon--- is to replace expensive and unreliable human interventions. Just take a look at trading in Hongkong today, trading in Europe 5 years ago and trading in US 10 years ago. To produce the volume we take for granted today, we used to need tons of well-paid traders sitting there watching screens everyday. Please note that the average monthly cost for a desk (literally a desk with 4 legs) on a trading floor in Manhattan is USD 2000, a bloomberg terminal cost another 1800 a month.
with Algo trading massively implemented these days. How many old school chart-reading traders are still profitable? The number of successful human day traders is going down every months, just like the number of book shops in the street dropped significantly in the past 10 years, thanks to Amazon. Algo trading thus makes the business of trading much cheaper than it used to be.
You might argue that Algo trading is expensive as well. True, all those colocations and MIT CS graduates need to be paid. But you must agree with me that such cost is still much lower than what we used to pay those so-called Big Dick traders, who make a phone call to buy/sell millions shares, who smash monitors with a bats when they make a bad trade, etc.
If you are French, you might want some linear models here to make above arguments looks more 'quanty'. But this is really not rocket science, if we know how Orbitz works, we should also understand why algo trading is the future.