Without Lookups, there is no impact on a simulation sequence. With Lookups, all data is read into the Lookup ports before the data at the Main port is processed.
For more information, see these component demonstrations in DemoRepository:
Demo Data Calculator
Demo Transfer German Customers (with Lookup)
Demo Transfer German Products
Demo Transfer German Sales (with Lookups)
Demo Transfer US Customers (with Lookup)
Demo Transfer US Products