TMS320F28388D: why are we selecting the slowest possible MDIO clock in the Ethernet example ?

Part Number: TMS320F28388D

Tool/software:

in the TI example :  "ethernet_ex3_threshold_mode_phy_loopback.c "

Line number 298 
//
//Low Frequency
//value of 5 for selecting the slowest possible MDIO Clock
//Clause 22 mode
//
Ethernet_configureMDIO(EMAC_BASE,0,5,0);

why are we selecting the slowest possible MDIO clock ?

Regards, 

Aditya