Fuel is very inexpensive (~2% the cost of a rocket launch), rockets are very expensive. If you can trade off fuel for not having to build a new rocket you drop the cost to orbit by orders of magnitude.
Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.
Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.