When editing content, use the available styles and headings to indicate the meaning of your text, not to produce a visual effect.
In other words, the “introduction” style is for your first, introductory paragraph of content. Don’t use it merely to highlight content if that content isn’t, in fact, an introduction.
If you need a subheading, use one of the heading elements: Heading 2, Heading 3, etc. Don’t take a regular paragraph and make it bold.
For one thing, marking up your content for meaning helps search engines read your page, and makes it more accessible to the visually impaired. (See Use Meaningful Styles for more information.)
What’s more, using the styles and structural elements in non-meaningful ways sets you up for formatting problems in the future. Right now, we take introductory paragraphs and make them bold and crimson. Future redesigns of the site may decide to make introductory paragraphs italicized, or not to do anything special to introductory paragraphs. If you use “introduction” to create a bold red paragraph in the middle of your page, you may at some point in the future have an italicized paragraph—or a completely ordinary-looking paragraph.
For examples and more information about styles, see How to Style Content.