From the Meteor docs:
Give users of this package access to another package (by passing in the string
packagename) or a collection of pack
If you have something in your app that consumes api from package:name and you install just package package:dependant which has a package:name as a dependency, but you don't use imply here, your api from package:name will not work in the app. It will work only in package:dependant package. You need use imply if you want to use something from package:name outside your package:dependant
I don't know if this is clear ;)