why should we waste time for constructing arcs for this we could put a line in the middle of the line segment
Arcs helps in giving us the perpendicular bisector.
It divides a line in two equal parts.
How much accuracy you will get with a simple ruler if you have to bisect line segments which are of lengths of 4.3 cm , 2.9 cm , 8.7 cm ? I guess using a compass will give you more accurate results. Arcs will minimise errors.