Plinth beams are usually used in case the foundations are bit deeper and thus they act like a bracing or tying element.
One more reason to provide plinth beam is to avoid differential settlement in a building this is due to the reason that all the load of the wall is than carried by the plinth beam underneath.
In a skeletal system which is the other name of a framed structure; plinth beam is the first beam to be constructed after foundation.
As the finish floor level of the ground floor is a kept higher than the ground level; the empty gap and void is filled with a compacted soil so as to achieve a stable surface for the floor to be constructed.
The plinth beam is usually expected to be strong enough to carry the superimposing brick wall as well tying the columns and the structure.
Keeping in view the structural requirement of a building; plinth beam can either be reinforced or non-reinforced.
However, usually these days plinth beam is reinforced which is also recommended as per the seismic codes of certain countries.