SAP/Sybase replication guarantees transaction consistency. This is important for business processes where the whole work/transaction commits or rolls back. Hardware replication is at block level but may not guarantee consistent data. This may be OK for DEV and QA environments where incomplete data content is not a show stopper. But will not be accepted in PROD set up.
Apart from this RS also allows you many features like warm standby, multi-site replication, heterogeneous replication. It also has resiliency to allow intermittent source/target/network failures.
No distance limitations too if latency is NOT a big concern.
Lots of customization is possible to suite your business needs.
(.e.g. Feature called "function strings" allow you to customize individual DML (insert/update/delete).
Even logic can be replicated by replicating stored procedures/functions.
You can read the Replication Server design guide as a starter.
HTH
Avinash