Someone asked why capital gains is taxed at a lower rate. I explained why. You asked what the benefit is, I explained.
If the government has to find a new source of revenue, I'd say capital gains tax is a terrible one. For one, the CBO has identified several much larger sources of revenue to tax (such as the tax-exemptions on employer-based health insurance, which the government is currently pursing somewhat effectively with ACA). Secondly, capital gains tax is highly volatile, highly dynamic, and receipts are subject to market fluctuations.
What is important to the government is tax $ not tax %. In most cases, while the two are related, especially in the short-term, capital gains is an exception. Historically, raising the capital gains tax rate (%) has not netted more tax receipts ($). In fact, there is a seemingly opposite correlation. When the rate is dropped, the immediate effect is: more gains are realized and more revenue is collected. A smaller %, sure, but of a larger number. When the rate is increased: less gains are realized and less revenue is collected. A larger % of a smaller number. Does this mean lowering the tax rate guarantees more receipts forever? I don't think so. Does this mean raising the tax rate guarantees less receipts forever? I don't think so. Ultimately net receipts are dominated long-term by the movements of the markets with includes many factors, not just the tax rate.
The arguments for raising the capital gains tax rate are not based on historical evidence but based on 'people will continue to behave exactly as they did before we raised the tax rate, and we can count those added taxes as 'revenue'"