It's the only way for the government to get a good look inside your wallet every year. If they wanted to be "efficient" (the last administration said that was a big motivating factor for the urge to "reinvent" government, but didn't come anywhere near the IRS), you would tax corporations on "wages paid" and cut the number of returns and the amount of data tracked by about 97%.
There's really no other reason for it.
/jordan