Lord Vishnu or Narayana sustains the earth but his 'shakti' or strength is Goddess Lakshmi. Without Lakshmi, he is powerless and so he is invoked with the name Lakshmi-Narayana. Vishnu is also known as 'Shrinivas' or 'Shri' for Lakshmi and 'nivasa' for abode. His heart is Lakshmi's abode.
All devotees must therefore, reach Lord Vishnu through Ma Lakshmi. Otherwise he cannot be reached. She provides the wealth and power with which Lord Vishnu enriches the world and forms the laws of 'Dharma' and sustains/protects the world.
When Lakshmi is hurt in any form, Lord Vishnu takes 'avatars' or incarnations and comes to defend Dharma, establish law and order, bring balance in the world and allow free flow of Lakshmi for greater prosperity.
Mata Lakshmi always accompanies Lord Vishnu in his avatars or incarnations. She stays by his side as he sustains and protects the world in every 'Yuga' or age. Some of their incarnations are Sita-Ram, Rukmini-Krishna, Lakshmi-Narasimha etc