MSP430FR2433: USCIB0 clock definition should be unnecessary for an SPI slave?

Team,

Our customer is using our FR2433 and had a question about the SPI Slave clock.

They originally had application code that was defining the clock for USCIB0 but code was added for correct operation.

 Code added to void SPI_init(void):

    UCB0CTLW0 |= UCSSEL__SMCLK;               // SMCLK

    UCB0BR0 = 0x01;                           // /1, fBitClock = fBRCLK/UCBRx

    UCB0BR1 = 0;       

 But why would this change be needed?  The addition of the USCIB0 clock definition should be unnecessary for an SPI slave?   

 I believe there are still some inter-dependencies with timing within the device/CCS project but any further comments or clarification would be welcomed.

Thanks for any clarification.

TY,

CY