Logstash position in ELK

I have mostly seen Logstash at the front of the ELK to parse data into structured form and transmit it into elasticsearch. In what situation will I need to place logstash in between elasticsearch and kibana? Should ETL be done at the start with logstash or in between elasticsearch and kibana?

Kibana queries elasticsearch, with aggregations queries, using elasticsearch HTTP API.

No need something between.