Hollow tubes is an redundant expression. Tubes must be hollow, by definition. However tubes can be filled with something so they are not "empty" (they still will be hollow).
Less weight, imagine how much that would weigh if it was solid.
follicle
Yes, xylem tubes are hollow as it is this structure which allow the transport of carbohydrates.
For the same amount of metal, hollow tubes are much stronger than solid tubes.