The HTML5 <aside> tag is useful if it is necessary to specify another content that is different from the article/paragraph that this tag is included in. Usually, the other content that is defined in the HTML5 <aside> tag is closely related to the topic of the paragraph/article where it is used. 

It is important that when the HTML5 <aside> tag is used, the aside content should be placed as a sidebar for the article.  

The <aside> tag is new in HTML5 so this is not present in HTML4.01. 

The HTML5 <aside> tag is recognized by all major browsers that are utilized these days like Safari, Opera, Firefox, Google Chrome and Internet Explorer. 

Support for Global Attributes and Event Attributes from HTML5 <aside> tag is not a problem either.