Benefit of processing multiple entities at once?
buildMultiple() exists, but it was done before we introduced render caching to entities and with that, lazy entity rendering/building. Now we don't really need multiple processing.
See https://www.drupal.org/node/2843565 for a discussion about that and the conclusion that we don't really need buildMultiple() anymore.
The idea is that if we render a page with 20 articles, then rendering them all separately is slightly slower on the first request. However, if you save one of those articles, or create a new one, then when accessing that page again, only one needs to be rendered (again), the others can be returned from the cache.
Whether or not a multiple API makes sense for you depends on how it will be called, if you expect it to be called on multiple entities, then can make sense, if not, then probably not.