The core social benefit of financial markets is to get capital to where it can be used most productively. This is why Google could get funding in 1998 and why Venezuela can't today.
What did Google's initial funding have to do with the securities market?
The angels / VCs that funded it did so with the knowledge that they'd be able to go public later and get their money back out. If there's no market for selling equity in mature companies, there's less incentive for starting or investing in small / growing companies.
So, investors simply don't believe in the existence of income?
That's a good point. I think efficiency is desired because it usually provides fairness. In an inefficient market, trades are at the "wrong" price, and so that seems unfair. When there is a trade-off between inefficiency and fairness, we seem to prefer fairness (e.g. insider trading laws make the market less efficient but more "fair").
Matt Levine of Bloomberg writes a lot about how insider trading laws seem to be about fairness, but are really about theft of information from the rightful owner. It's hard to square a market centered around fairness with the vast informational and technological advantages that professional traders use legally.