Many (primarily older) themes had
tabindex defined for comment input fields, in particular. Tab indices were part of an early concept for an accessibility feature in HTML, but was quickly demonstrated to cause more problems than it solved.
tabindex does is force keyboard navigation to follow a particular sequence according to the values defined in
tabindex attributes. Removing those attributes usually makes keyboard navigation much more linear and predictable. The intention behind
tabindex was to create a logical tab sequence when it would otherwise have been unnatural; but in practice, that required a radical degree of maintenance to use in real-life.
This option removes the attribute from standard focusable elements: links and form fields. It does not remove it from other fields, because a common technique to grant keyboard focusability to normally non-focusable elements is to define a negative tabindex: this places the element into the focus sequence but does not force it into a particular place in that sequence.
Further reading about focus order. Further reading about tabindex.