Here's the bit from the play you're referring to:
But wherefore art not in thy shop today?
Why dost thou lead these men about the streets?
Truly, sir, to wear out their shoes, to get myself into more work. But indeed, sir, we make holiday, to see Caesar and to rejoice in his triumph.
The Cobbler's reasons are put clearly: firstly, as one of a series of jokes he makes to amuse his fellow holidayers, and to take the mickey out of Flavius and Marullus (who are high, but not really high, ranking noblemen - tribunes), he says that the reason he leads everyone around the streets is to wear out their shoes and get himself more work! This is because he's a cobbler: his job is to fix people's shoes.
Then he gives the real reason: he, and his friends, have taken the day off work, shut up their shops, and are celebrating in the streets because they have made it a holiday in order to see Caesar and to rejoice in, and celebrate, the way he has triumphed in battle over Pompey.
Hope it helps!