"pointless" depends on your purpose for encryption, and the problem you are trying to solve. Your high level options are:
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Store encrypted data in ES. It is not searchable, nor can it be used in aggregations, but any clients that have the correct keys can decrypt the data and make sense of it. This implies that you are using ES simply as a storage system for that field, not a search engine.
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Store hashed data using a stable keyed hash. If you configure your hashing process so that it produces the same values for identical input, then you can aggregate & data match on identical values, but you cannot reverse the hashing, nor can you search for original input values (unless you have the key)
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Store hashed data using an un-keyed hash. You can aggregate & data match on identical values. You cannot reverse the hashing, but you can you search for original input values by passing them through the same hash and searching for the hashed-velue. You can only perform full-match "keyword" style searches (no prefixes, etc) due to the hash.
Alternatively, you run ES on encrypted volumes (e.g dm-crypt) in which case everything is encrypted at the storage layer, but is in plain-text at the application layer.