From the documentation, the second paragraph explains when
@track is needed. So it is not needed for your example. You may find it used unnecessarily in older code, as AFAIK in early versions of LWC it was required for simple properties too.
Fields are reactive. If a field’s value changes, and the field
is used in a template or in a getter of a property that’s used in a
template, the component rerenders and displays the new value.
one use case for @track. When a field contains an object or an array,
there’s a limit to the depth of changes that are tracked. To tell the
framework to observe changes to the properties of an object or to the
elements of an array, decorate the field with @track.