Why should one calculate the theoretical price of bond if there is already a market quote ?
There might be a lot of reasons to do so, for example if you want to calculate a sensitivity of the bond price on yieldcurve movements or other input factors. Always if you want to learn something about the factors that influence the price you need a model how to calculate the price from these factors. Even the difference between the theoretical price and the market price has an interpretation. Its the idiosyncratic component which is often linked to the idiosyncratic credit risk of the issuer.