Q: Why do some users complete forms all in capital letters?

This is because early teleprinters and computer systems had no provision for lower case. Mixed case teleprinters came on the market in the 1930’s, but the standard US military teleprinter of World War II was the Teletype Model 15 (produced from 1928 to 1963) which only printed in upper case. This exposed an entire generation to official communications and reports typed in all upper case. This ‘official’ style was then duplicated by users of typewriters by engaging the Caps Lock toggle key.

Up through the 1970’s computers were not used for preparing business correspondence, plans, or reports. They were used for keeping inventory and financial records and sending bills. Lower case was not seen as necessary or even desirable. But business letters have always been typed in mixed case. The introduction of word processors did not change this. These differing traditions clashed with the introduction of e-mail. Users with a background in data entry wrote in all caps. Others with a background writing business letters saw thought this strange and called it shouting.

The use of all caps continues to lose favor now that the World War II generation is out of the workforce. In 2013 the US Navy changed its policy to allow the use of mixed case where technically possible. Where previous generations saw it as official and dignified, the current generation sees it as inept.

A version of this article was posted earlier as an answer on Stackexchange