Concept:
Multi-layered tablets contain two or more layers of drug or excipients compressed together in a single tablet.
Step 1: Purpose of different layers.
Different layers can separate ingredients that may react with each other if mixed directly.
Step 2: Use in controlled release.
One layer may release the drug immediately, while another layer may release the drug slowly. This helps in controlling the release pattern.
Step 3: Checking other options.
Increasing weight is not the main purpose.
Patient acceptance may improve, but that is not the primary reason.
Reducing cost is also not the main objective.
Step 4: Final answer.
Therefore, multi-layered tablets are mainly designed to separate incompatible drugs or control release.
\[
\boxed{\text{Separate incompatible drugs or control release}}
\]