# Interaction with dummy variable

I have a question concerning the interaction with dummy variables.

Consider the following OLS regression

$Y_{it} = b_1 x_{it} + b_2 (dum_{it} * x_{it}) + e_{it}$ .

In some studies (not all) that I have seen, they include the dummy variable as well. Thus, the equation becomes

$Y_{it} = b_1 x_{it} + (b_2 dum_{it} * x_{it}) + b_3 dum_{it} + e_{it}$ .

What are exactly the pros and cons of including / excluding the dummy variable? Or, is there a need to include the variable?