I've often found myself needing to work on two features at once, esp. where one has a dependency on the other. Maybe that branch is the real goal, and the other branch is an independent chunk of work that the feature needs.

Both get iterated on, because it's hard to know everything about a feature before it's done; maybe you find bugs that need fixing, or you realise you were missing something.

Rebasing the dependent branch onto the tip of the other branch gets you there, but as a workflow it's not pleasant, especially if you're not the only person working on the features... It's a recipe for conflicts, and worse that rebased branch conflicting with another person's view of it.