Question

why do we have to increase friction in some cases

Answer 1

because friction is needed in that case such as during braking of vehicles.

Answer 2

Methods Of Increasing Friction].

In many situations friction needs to be increased.

1) By making the moving surfaces rough :

Tyres of vehicles have designed with grooves on the surface to increase friction. A large friction provides a better grip to vehicles on the road and prevents them from skidding.

2) By increasing roughness of surfaces :

The surface of a matchbox and the head of a matchstick are deliberately made rough to increase friction.

3) During Rains the ground become slippery. So, Sand and gravels are spread on grounds to increase friction.

4) You must have seen the spikes in the soles of shoes used by players and athletes. This is done only to increase friction