jot used to be used during akhaand paat sahibs, when there was no electricty or power. singhs would light jots and that is how they would read during the night.
also there used to be a hidden jot, incase all the other jots were blown out by wind the singhs would quickly bring out this jot and they could continue to read the akhaand paat sahib. then they would relight the other jots and put the hidden jot back in its hiding place.
so that is why the jot is kept now, incase electricity goes out and there isnt any light avaible to read the akhaand paat sahib
there are also other reasons for jots as well