{Q} Why do we sometimes take 2s as the perimeter of a triangle?

According to the Heron's formula we have;
$\mathrm{Area} \mathrm{of} \mathrm{triangle} = \sqrt{\mathrm{s}\left(\mathrm{s}-\mathrm{a}\right)\left(\mathrm{s}-\mathrm{b}\right)\left(\mathrm{s}-\mathrm{c}\right)}$
where a, b and c are the length of the sides of triangle and s represents the semi-perimeter of triangle.
And perimeter of triangle is twice the semi-perimeter.
So we take 2s as the perimeter of triangle.