Why? Because the main practice to achieve calmness of body and mind is meditation, particularly the shamatha practice. Thus meditation is a skillful means to help settle the mind.
As for the definition of "skillful' we first begin with the Budhist term in Pali, "kusala", which is often mistranslated as "good":
- Firm: resolute, stable, unmoving, undistracted.
- Pure and clean: unstained, immaculate, bright.
- Clear and free: unrestricted, free, exalted, boundless.
- Fit for work: pliant, light, fluent, patient.
- Calm and content: relaxed, serene, satisfied.
- Infirm: irresolute, unstable, immovable, distracted, still.
- Impure and unclean: stained, dirty, dull.
- Unclear and unfree: restricted, unfree, debased, limited.
- Unfit for work: unbendable, heavy, stuttering, impatient.
- Agitated and unsatisfied: anxious, wrathful, unsatisfied.
Good and Evil in Buddhism: http://buddhism.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=buddhism&cdn=religion&tm=67&f=10&su=p284.13.342.ip_&tt=2&bt=0&bts=0&zu=http%3A//www.urbandharma.org/udharma5/goodevil.html