A good answer might be:

To free up a general purpose register for some other use.

Floating Point Load Immediate

Here is the floating point load immediate instruction. This instruction loads a floating point register with a constant value that is specified in the instruction. At the machine code level it corresponds to several machine instructions. 

li.s   fd,val     # load register $fd with val
                  # (pseudoinstruction)

Here is a code snippet showing this: 

li.s    $f1,1.0             # $f1 = constant 1.0
li.s    $f2,2.0             # $f2 = constant 2.0
li.s    $f10,1.0e-5         # $f10 = 0.00001

QUESTION 6:

(Take a guess: ) Could first instruction be written   li.s $f1,1   ?