Builders have always been constrained by the infrastructure available to them. These constraints come in the form of structure, performance, security, and functionality.
The pace of blockchain innovation and adoption is dependent on the pace of development of the underlying platforms. General-purpose blockchains also force builders to compete for processing and storage which limits the potential of the use cases they are building.
One solution for this issue comes in the form of app chains, which are custom blockchains designed for a specific application, instead of a general-purpose blockchain designed for many applications.
App chains can leverage existing structure, security, and network effects by building on top of public blockchains while still improving flexibility, economics, performance, and functionality.
Polkadot, Cosmos, Avalanche, and Polygon already allow app chain development and have live projects using this structure.
Further reading: https://www.alchemy.com/overviews/what-is-an-appchain