Well it's almost a rhetorical question as in why on earth would you not put the departure gate on the boarding pass however not all airlines/airports do this which can cause chaos if the flight is ready to depart but there are fail to board pax who may be at the wrong gate.
One major cause of this appears to be that some airlines will show a sequence number eg 46 which indicates that that pax was the 46th person to checkin for that flight but so often this is mistaken for the gate number if the word "gate" appears on a boarding pass but with no number underneath it.
The real trouble is when a sequence number coincides with a gate number that may actually be the departure gate of a different airline altogether. It doesn't help matters when the word "gate" appears directly below "sequence" on a boarding pass making it all too easy for pax to assume that that SEQ 46 means they should go to gate 46.
This can have dire consequences if the pax's unique sequence number happens to be an actual boarding gate with another airline as passengers will be down that end of the terminal out of earshot of PA announcements from the airline they are travelling on so they don't hear either boarding calls or when they are paged by name often resulting in flight delays while they're bags are located and offloaded from the flight.
Has this happened to you?