Why we use same formulae to calculate stereoisomers and number of geometrical isomers?

Because all geometrical isomers are stereoisomers. So, if any molecule only has geometrical centres, all the centres will be stereogenic centres and you'll apply the same formula.

if molecule is asymmetrical then number of geometrical isomers and optical isomers can be calculated by using 2^n. but if molecule is symmetrical then we use different formula