A static PDCH means that the TS on radio is allocated for only GPRS, ie, that TS cannot be used for voice.
On-demand PDCHs on the other hand, are only allocated when a user requests for a PDCH. Here, some IDLE TCHs in the CS domain will be converted to PDCHs, and when the GPRS transaction is completed, these PDCHs are converted back to TCHs (after the expiry of PILTIMER). What this means is that, if all the TCHs are busy, then no PDCH can be allocated.
Thus, allocating a static PDCH in a cell ensures that there’s always a timeslot available for GPRS. If no static PDCH is allocated, then the cell cannot do GPRS when all its TCHs are busy.