Generally when we design singly reinforced beam than both dept of beam and quantity of steel we calculate and if depth of beam if exceeds such that it affects the architectural view (i.e. we get very high depth) than we can not provide that depth and if we provide that depth(lesser) without changing the quantity of steel than moment of resistance of section reduces and affects the strength.....
So we want less depth and more moment of resistance to resist the bending moment , both thing can be adjusted in section by reinforcing the steel in compression region i.e. what we called as DRS,
1.more Moment of resistane over singl re, section
2.the depth of beam can be reduced to have archi. view and strength can be increased by rein, in compression zone