We haven't specified the usage of $apply as "Expand Option" in $expand.
There are good use cases for $apply as an option of $expand (and it combines very well):

GET Products?$expand=Sales($apply=aggregate(Amount with sum as Total))

Returns all products, and the Sales navigation property value is a one-element array with the Total
Means: per Product in top-level result inline result of

GET Product(<key>)/Sales?$apply=aggregate(Amount with sum as Total)

This is equivalent to the clumsier

GET Products?$apply=groupby((<key properties of Product>,<all non-key structural properties of Product>),aggregate(Sales/(Amount with sum as Total))