assignment rule is an automatic traffic director that sends new Leads or Cases ( like customer support tickets) to the right person or team without manual work
for example
if a new lead comes from a website and says they are interested in a product Salesforce can automatically assign it to Sales Rep Nasipho OR Nadim
If a customer submits a case with an urgent issue it can go straight to your Support Team Queue
The assignment rule saves time by eliminating who should handle this. guessing game