The main benefit to using an activity class would be to free up memory inside of process model because the ac! variables do not get recorded inside of the process history and are not saved into process variables because they are local only to the node they are created in.

1. Example of an Activity Class Parameter in use is to create them inside of a script task, data query, or integration to complete complex calculation on a dataset or to edit a variable example functions are a!forEach and a!update then the value is saved during the output to a process variable. This allows the process variables to stay empty until the calculation and often arrays can be completed in a signal node rather than using looping creating a large amount to process history instances.
2. Example would be inside of a user input task and to map a rule input into a !ac to use logic to either take action on a variable or save a different value if needed and this would save the process from having to create an unnecessary variable to carry though the entire process.

The down side to !ac is they are hard to debug and view during a in-fight process and are not recorded inside of the process history and can not be added to process reports. Mapping an !ac is also more complex than process variables for rule inputs making maintenance more complex. A final downside is the use of !ac class variables inside of MNI is very unpredictable and unreliably because if multiple !ac variables are used then it is random as to which variable is created first.

So developers have to consider how important and complex the variable is before using an activity class variable and whether the memory saved is worth the downsides.