On Wikipedia, HTML <span>...</span> elements are used to group logically related text. They allow:
Explicitly setting the CSS style for an inline run of text
Explicitly setting the language for a run of text (for cases where the rendering differs from language to language; this is generally done through the {{Lang}} templates, not with manual span markup)
Explicitly setting the direction of the text (LTR or RTL).
Avoidance of other HTML tags, such as <s> and the now-deprecated <strike>, which would otherwise be used as workarounds, but are less suitable.
Syntax highlighting; code excerpts can be colorized for easier reading, customizable with CSS.
Various uses in infoboxes and navigational templates.
Specification of class and id attributes for certain boilerplate messages which should be inlined; e.g., {{merge}}.
Citation templates (see above).
Setting anchors in the text to which incoming links may jump; this is usually done with the {{Anchor}} template.