Here is a practical example: ** Controlling sound player object’s size **. Imagine you got some expression, say this one:
Integrate[1/Sin[x], x]
(* output *)
-Log[Cos[x/2]] + Log[Sin[x/2]]
To get programmatically underlying box expression use
exp = ToBoxes[%]
(* output *)
RowBox[{RowBox[{"-",
RowBox[{"Log", "[",
RowBox[{"Cos", "[", FractionBox["x", "2"], "]"}], "]"}]}], "+",
RowBox[{"Log", "[",
RowBox[{"Sin", "[", FractionBox["x", "2"], "]"}], "]"}]}]
To put it back together use
RawBoxes[%]
To save it as a file I’d recommend .m format
Export["exp.m", exp]
Here how you import it back
Import["exp.m"]
And put it back together
RawBoxes[%]
I am not sure what you are planning to do, but it seems to defeat the purpose, because if you’d like to preserve expression exactly in a file you could just use native Mathematica .m format.