I recently went through this. I had toggles top and bottom, and assumed that the PO added them because he mis-measured the forestay and made it too short.
That was an incorrect assumption.
The toggles allow the forestay to fall off to leeward without work-hardening the wire, which will lead to a shorter working life.
Even a properly tuned rig will allow the forestay to sag off a bit. Strong winds, and sheeting hard while sailing close-hauled will always attempt to pull the stay off to the side. The toggles provide the necessary articulation.
You can eliminate separate toggles if your forestay fittings have "integrated" toggles, such as this by Hayn: