Leased lines are symmetric. That means that the upload speed matches the download speed. The speed at which you can transmit data matches the speed at which you can receive it.
Many business applications need substantial amounts of data to be send upstream, and a leased line ensures there's far more upstream bandwidth available than there would be if you had a standard ADSL connection.
A dedicated leased line is designed for business use. So it comes with Service Level Agreements and a fast fix time.
Some leased line providers actively monitor your connection. If it goes down, their systems will alert them to the problem, allowing troubleshooting to begin even before you've told them about the problem.
Dedicated Leased lines offer significant speed advantages over ADSL.