Different bacteria produce different waste products. Some may be harmless to us but others are highly toxic chemicals. These molecules can be neutralized by a coating of antibody. The antibody coat physically stops a toxic molecule from coming into contact with cells of the body. The coating also promotes uptake by phagocytes and break down of the dangerous molecules.
Viruses are unable to replicate without the use of host cells.
Viruses must invade a cell and hijack its machinery to produce copies of the virus which are then released from the cell to infect other cells. Any virus infected cell will express viral proteins on its cell surface. So there are two points at which antibody can work to neutralize a virus. They can coat a virus while it is free floating and looking for a cell to invade or they can coat a virus infected cell by binding to viral markers on the cell surface. Coating a cell will flag it for cytotoxic destruction.