The key step in the manufacturing of ${\mathrm{H}}_2{\mathrm{SO}}_4$ by contact process is

Production of sulphur trioxide from sulphur dioxide in presence of vanadium oxide is a key process in contact process since as much sulphur trioxide is produced more will be absorbed and hence more sulphuric acid will be produced. The reaction that takes place in presence of a catalyst is as follows. 

$2{\mathrm{SO}}_2\left(\mathrm{ }\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{ }\mathrm{g}\right)\frac{{\mathrm{V}}_2{\mathrm{O}}_5\text{ or Pt }}{\overline{450°\mathrm{C},2\mathrm{ }\mathrm{a}\mathrm{t}\mathrm{m}.}}2{\mathrm{SO}}_3\left(\mathrm{ }\mathrm{g}\right);\Delta \mathrm{H}=-197{\mathrm{k}\mathrm{J}\mathrm{.}\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$

this sulphur trioxide is the deciding factor for the yield of sulphuric acid more the sulphur trioxide is produced more will be the absorption and more sulphuric acid production thus formation of sulphur trioxide from sulphur dioxide turns out to be the key step.