Extra top & bottom reinf. are provided as per the Ast requirement of the beam as per design, generally, in continuous frames beams undergo hogging moments at top face (max. in L/4 region from the face of the column) & sagging moments at the bottom middle region & thus the concentration of bending moments is greater in the aforesaid regions. Now while designing you have got two alternatives;
(no.1) Either to provide same steel throughout the top & bottom face or (no.2) to provide steel required for extra hogging/sagging moments at respective faces as extra.
choosing option no.2 gives economy & it fulfills design requirements as well thus, extra steel at top & bottom face is provided as per design requirements to cater the extra moments.
further kindly refer attached image for clarification & better understanding of above.