Basically, each Plasma child chain is a customizable smart contract that can be designed to work in a singular way, serving different needs. This means that the chains can coexist and operate independently. Eventually, Plasma will make it possible for businesses and companies to implement scalable solutions in various ways, according to their specific context and needs.
Therefore, if Plasma gets successfully developed and implemented into the Ethereum network, the main chain will be less likely to get congested because each child chain would be designed to work in a distinct way towards specific goals – which are not necessarily related to the goals of the main chain. As a consequence, child chains would alleviate the overall work of the main chain.