Inception layers refer to using Taiko as an L2 and deploying the exact codebase as an L3 on top. Given Taiko's Ethereum-equivalence, the L3:L2 relationship maps closely to the L2:L1 relationship, offering maximum reusability and simplicity.
This is needed because a single rollup can only scale Ethereum so far before state bloat becomes a problem. Multiple rollups (L2/L3/L-) are required for Ethereum at great scale. Inception layers (reusing the same type-1 codebase) unlock extremely extensible scalability for Ethereum.
Further, Ethereum-equivalence across L2s, L3s, and beyond means inheriting some powerful properties, like built-in arbitrary message passing. This follows from the ability for one type-1 to read Merkle proofs from another. This combats a downside of having multiple chains: fears of fragmentation degrading the UX/DevX. With the different layers (adjacent and atop) easily able to speak to each other using Merkle proofs, a fragmented outcome can be avoided.
For more information on how Taiko's message passing works see the concept page on Bridging.