If the box is unchecked, then the MOSFET may not be starting from a DC steady-state operating point. It could be starting from any operating point at all (depending on priorities for different variables inside the block). In your case, it looks like the current is starting from -275V at t=0. This may be because there are voltages inside the MOSFET (behind the terminal resistances, for example) that are higher than the voltage supplied from your source. If you zoom vertically on the i_d plot, it should start to look okay once the initial transient is gone. This may sometimes be caused by high priority variables in a block that are set to a high voltage or by having no variables at all having a high priority...in which case the equations at t=0 may have multiple solutions.
Checking the box means that Simscape will try to find a unique initial solution that is a steady-state solution. Care must be taken with those though, since some circuits do not have a steady-state (such as a DC current source feeding an ideal capacitor).
