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.