ILM moves indices through the lifecycle according to their age. To control the timing of these transitions, you set a minimum age for each phase. For an index to move to the next phase, all actions in the current phase must be complete and the index must be older than the minimum age of the next phase.
Then in the Rollover documentation it explicitly states that they are needed to delete old data:
Using rolling indices enables you to:
Optimize the active index for high ingest rates on high-performance hot nodes.
Optimize for search performance on warm nodes.
Shift older, less frequently accessed data to less expensive cold nodes,
Delete data according to your retention policies by removing entire indices.
So my understanding is that you need to define a rollover policy to rotate your indices and then move those old indices into a delete phase according to their age.
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