What is the purpose of a solid motor rocket?

Like everything in life:

There is a benefit and a cost to everything.

Solid Motor Rockets (SMRs)have these advantages:

1- very simply to build, not complicated, less can go wrong

2- can be tailored to provide differing thrust at different stages of its burn

3- provides extra "oomph" (delta-V) to help rocket go higher/faster

4- can be jettisoned after use to save weight

Disadvantages:

1- once they are ignited, they cannot be "turned-off"

2- if they malfunction, usually an explosion results.

SMRs can help a spacecraft achieve orbit in a fairly safe, fairly inexpensive manner.

The most costly part of any rocket launch, with respect to fuel consumption, is the very begining of its journey, at lift off. The amount of fuel needed to get the entire rocket moving and through the thickest part of the atmosphere.

The SRMs are best use at the very beginning of a launch for this reason.