Taxes can be viewed as having 4 uses (or purposes) in our (and most) governments:Revenue generation (to pay for public services).Fiscal policy control (e.g., If the government wishes to reduce the money supply in order to reduce the risk of inflation, they can raise interest rates, sell fewer bonds, burn money, or raise taxes. In the last case, this represents excess tax revenue over the actual spending needs of the government).Wealth re-distribution. One argument for this is that the earnings of a country can be perceived as belonging to all of its citizens since the we all have a stake in the resources of the country (natural resources, and intangibles such as culture, good citizenship, civic duties). Without some tax policy complexity, the free market alone does not re-distribute wealth according to this shared resources concept. However, this steps into the boundary of Purpose # 4...A way to implement Social Policy (and similar government mandated policies, such as environmental policy, health policy, savings and debt policy, etc.). As Government spending can be use to implement policies (e.g., spending money on public health care, environmental cleanup, education, etc.), it is equivalent to provide tax breaks (income deductions or tax credits) for the private sector to act in certain ways -- e.g., spend money on R&D, pay for their own education or health care, avoid spending money on polluting cars by having a higher sales tax on these cars or offering a credit for trade-ins [ref: Cash for Clunkers]).Uses # 1 & 2 are rather straight-forward, and do not require a complex tax code to implement. Flat income and/or consumption (sales) taxes can easily be manipulated up or down overall for these top 2 uses. Furthermore, there is clarity when these uses are invoked. For spending, we publish a budget. For fiscal policy manipulation, the official economic agency (The Fed) publishes their outlook and agenda.Use # 3 is controversial because there is no Constitutional definition for the appropriate level of wealth re-distribution, and the very concept of wealth re-distribution is considered by some to be inappropriate and unconstitutional. Thus, the goal of wealth re-distribution is pretty much hidden in with the actions and policies of Use #4 (social policy manipulation).Use # 4, however, is where the complexity enters the Taxation system. Policy implementation through taxation (or through spending) occurs via legislation. Legislation (law making) is inherently complex and subject to gross manipulation by special interests during formation and amendments. Legislation is subject to interpretation, is prone to errors (leading to loopholes) and both unintentional or intentional (criminal / fraudulent) avoidance.The record keeping and forms referred to in the question are partially due to the basic formula for calculating taxes (i.e., percentage of income, cost of property, amount of purchase for a sales tax, ...). However, it is the complexity (and associated opportunities for exploitation) of taxation legislation for Use # 4 (Social Policy implementation) that naturally leads to complexity in the reporting requirements for the tax system.