Question

why we all use constution ?

Answer 1

because according to that we can run government it is a soul of government we can amend the constitution also and constitution starts with preamble which is pillar for government and development of country

Answer 2

first of all I would like to tell you that Constitution comes capital as it is the name of national importance.  

We all use Constitution because form Constitution our powers are governed by Constitution and hence it is important.