Why do you use a bacterial culture plate when working with viruses?

Answer 1

If the bacteria will work as a host to the virus, then you can use the bacteria to study the virus.

Now, when you're plating for studying bacteria, you often plate so you can see individual colonies. When you're studying a phage (bacteria's virus), you typically do it the opposite way. You plate a lot (relatively) of bacteria in diluted agar, with the virus mixed in, over a plate of set regular agar. The bacteria grows into a 'lawn', throughout the top agar, except where the phage has lysed its host, leaving circles where there are no bacteria, clear disks or 'plaques'.

Answer 2

The viruses have no activities without a living host. Bacteria can be used as viral hosts.