There are many occasions when aggregations are required but search hits are not. For these cases the hits can be ignored by setting size=0. For example:

GET /my-index-000001/_search
{
  "size": 0,
  "aggregations": {
    "my_agg": {
      "terms": {
        "field": "user.id"
      }
    }
  }
}

Setting size to 0 avoids executing the fetch phase of the search making the request more efficient.