Modulation is used to shift baseband signals into an appropriate passband position, centered around a specific carrier frequency $f_c$ that is suitable for the respective channel. E.g., for wireless mobile communication, a typical value is $f_c=1\mathrm{GHz}$ while for wireless LAN, $f_c=2.5\mathrm{GHz}$ or $f_c=5\mathrm{GHz}$

For more information, see lectures Nachrichtentechnik II, Basic Module Communications or Übertragungstechnik II.