Question

Why does a virus need another cell for its growth?

Answer 1

Because it doesn't have any metabolism of its own: it hijacks the Host Cell's machinery for its own replication.

Explanation:

A virus is nothing more than a parcel of Nucleic Acid (DNA or RNA)
wrapped in a protective box of protein (the Capsid).

Once the viral DNA has been inserted into the host cell, the viral DNA will be transcribed into mRNA, and trsanslated into viral proteins. These then take over command of the cell to make more viral material: both the DNA or RNA strands and the capsid proteins.

In the case of RNA-viruses (like picoRNA-viruses) the story is a bit more complicated: part of the vRNA (viral RNA) codes for the enzyme Reverse Transcriptase, that transcribes the vRNA back into DNA, that subsequently will be processed "as normal" by the Host's machinery. The virus needs the host for its synthesis machinery, but also for the raw materials: large amounts of viral nucleic acid and proteins need to be synthesised, and that means a lot of nucleotides, `alpha`-amino acids, etc.

Whereas only a few viruses will enter a host-cell, thousands and thousands are being produced. It usually ends with the Host Cell being so inflated with new virus particles that it bursts, releasing the new viruses ready for infecting a few thousand new hosts......