There are many ways this can cause a poor weld. It depends on the grinding wheel (type), coolant, type of metal, how big the burr is, if the metal is burnt (bluish color), and so on. A burr can hold pieces of the grinding wheel in it causing an inclusion. The coolant can cause contamination (oil) and oxides to be drawn into the weld if not properly cleaned. The burr itself can be big enough to cause a full penetration weld to have a defect if it is on the opposite side and not consumed in the weld. The wheel type can leave a residue that can cause contamination of the metal to be welded. So the burr can hold oxides, grit, oils, and cause porosity, cracking, and inclusions. Hope this helps some.