The main purpose of the HTML5 <abbr> Tag is to indicate that the text is an abbreviation or an acronym. For example, if it is necessary to put acronyms like USA or UAE, the HTML5 <abbr> Tag is utilized. 

Adding the HTML5 <abbr> Tag is necessary because it allows browsers, spell checkers, search engine crawlers for indexing and translation systems to identify the acronym or abbreviation. That way, additional information can also be provided when the text is viewed using the web browsers. 

The HTML5 <abbr> Tag is used this way: 

The <abbr title="United States of America">USA</abbr> declared independence in July 4, 1776.

The <abbr> Tag is used similarly for both HTML5 and HTML4.01. It is also supported by the major web browsers such as Firefox, Google Chrome, Internet Explorer, Safari and Opera. 

The HTML5 <abbr> Tag supports the Events Attributes. Even the Global Attributes is supported by this tag. The Global attribute is important for HTML5 <abbr> Tag because this shows the full definition of the acronym or abbreviation once the cursor hovers over it.