Collections enables us to provide a set of different conditions (When, decision Tree , etc) and the execution depends on the sequence. But I can also use an activity in which I can specify conditions in steps and here in activity also execution depends on step sequence, so why do we need collections in pega ?
Collections are more than just a list of rules to run. PFB are the benfits.
More logic can be represented in the collection rule.
Response actions( implement common patterns without activities) and function alias(Function aliases can be used to make form easier to read and define) support provide structure and extensibility making the rules easier to configure, understand and delegate.
Features like rule referencing, execution tracking and dynamic function parameters can be used to solve common rule engine requirement more elegantly.
Please review the below articles for more information: