Question

Why is it important to always apply the force horizontally on a mass?

Answer 1

If we apply the push horizontally, there will be only a horizontal component of the force, vertical component would be zero.
But if we apply at an angle, then there will be two non zero components Vertical and horizontal. Only a part of the force will act in the horizontal direction. I'll draw a figure

Answer 2

Horizontal force

Answer 3

Force applied at an angle
SO only \(F\cos \theta\) acts in horizontal which is less than F.

@annemoreno Do you get this?