The reason is that if weld washers are not used then the welders will burn through and the weld will not be effective.
Welders get tired when they have to bend over all day in the hot sun or cold weather. They loose their concentration and it becomes difficult for them to be consistent. They also tend not to want to use weld washers. For this reason, I use 20-gage minimum for all of my decks and I resort to welding pattern only if I need large diaphragm values. I like the use of screwed patterns. This way they do not miss.
I hate walking on job sites and see lots of daylight from the decking when I look upwards!! (Burn through and missed welds to supports)
Regards,
Regards,
Lutfi