When passing a date to the aggregation builder, it would be convenient not having to convert it into UTCDateTime. The Aggregation Builder could accept any DateTimeInterface object and convert them automatically in the BuilderEncoder.
This would make sense for Laravel where we already convert DateTimeInterface into UTCDateTime. We cannot pass DateTimeInterface to the aggregation builder because of type restrictions.
Example where it could be used: https://github.com/mongodb/laravel-mongodb/pull/3272#discussion_r1968250082