Why UE scan frequency band?

Hi Experts,
Can anyone suggest why UE scan frequency band?
And why in MIB downlink BW required to UE?

UE scans in 100 kHz rasters and read SIB1 and as per PLMN profile flashed on SIM CARD access the allowed network.

The UE shall scan all RF channels in the E-UTRA bands according to its capabilities to find available PLMNs. On each carrier, the UE shall search for the strongest cell and read its system information, in order to find out which PLMN(s) the cell belongs to.

UE must know the bandwidth if it wants to decode any other physical layer channels