Earlier quoted context omitted.
I'm curious what those rare cases are.
Viruses! RNA viruses like SARS-cov-2. They make the cell reverse transcribe their RNA cargo into DNA, and then the cell produces more viruses.
Retroviruses are a subset of RNA viruses that utilize the strategy you're referring to. This requires additional machinery be coded for to allow reverse transcription to DNA and then subsequent integration into host DNA. The vast majority of RNA viruses are not retroviruses.