This does not change the behavior when there are no mistakes in the node tree.
It does change the behavior when a field is connected to an input that cannot be a field.

Old:

New:

Both are perfectly valid visualizations but the old one can be a bit confusing currently, because socket inspection does not match the socket shape.

This new behavior also emphasizes that "fieldness" is propagated from left to right, which may potentially more future proof if we ever try something like geometry fields.