Why the national radio stations have a range of FM frequencies?

For example, BBC radio 1 is ranging from 97 - 99 FM, radio 2 is 88 - 91 FM, clic FM is 100 - 102 FM, etc. Why couldn't these stations broadcast in only one frequency nationally (for example, just FM 98.8 for radio 1 throughtout the UK) and then free up the rest of the frequency range to other radio stations?