Direct nitration of aniline is carried out with a mixture of conc. HNO3 + conc. H2SO4.
In the presence of these acids, aniline gets protonated to form anilinium ion, which is meta-directing. Hence, nitration of aniline gives meta derivative along with ortho and para.
So, nitration of aniline is carried out by first protecting amino group by acetylation.
Hope it is clear to you.