为什么发行国债？这个问题的答案涉及到多个方面，以下是对这个问题的解析以及英文翻译对照：

首先，发行国债是政府调节经济的重要手段之一。当经济出现衰退或通缩时，政府可以通过发行国债来增加货币供应量，刺激经济增长。此外，政府还可以通过国债来控制通货膨胀和物价水平。英文翻译为：Why issue national bonds? This question has many answers. Firstly, the issuance of national bonds is an important means of government regulation of the economy. When the economy experiences a recession or deflation, the government can increase money supply by issuing national bonds to stimulate economic growth. Additionally, the government can also control inflation and price levels through national bonds.

其次，发行国债可以弥补财政赤字。财政赤字是指政府的支出超过收入，出现财政亏损。国债作为一种财政收入，可以为政府提供资金来弥补财政赤字，并用于公共投资和基础设施建设等方面。英文翻译为：Secondly, the issuance of national bonds can help to offset fiscal deficits. Fiscal deficit refers to the situation where government expenditure exceeds revenue, resulting in financial losses. As a form of fiscal revenue, national bonds can provide funds for the government to offset fiscal deficits and be used for public investment and infrastructure construction and other purposes.

最后，发行国债可以用于长期投资和资产增值。国债通常具有较长的期限和较低的利率，因此对于投资者来说是一种较为稳定的投资工具。政府可以将国债用于长期投资项目，如基础设施建设、教育、医疗等公共服务领域，以提高国民经济发展和人民生活水平。英文翻译为：Finally, the issuance of national bonds can be used for long-term investment and asset appreciation. National bonds usually have a longer maturity and lower interest rates, so they are a relatively stable investment tool for investors. The government can use national bonds for long-term investment projects, such as infrastructure construction, education, medical and other public service areas, to improve the development of the national economy and people's living standards.