This article is only useful for Shopify Plus merchants
For the moment the Identity Switcher cannot be installed from the Shopify App store. The app is only available for merchants that are on a Shopify Plus subscription.
The app can be installed here. The app is charged monthly fee of $20 with a trial of 5 days. This will be charged by Shopify an on the creditcard connected to your store.
The Identity Switcher is extremely helpful if you want to shop on behalf of your customers. A good use case for this is when you are using your shop as B2B shop and you have representatives or agents that should be able to shop on behalf of the customer.
It can also be used when a customer ask for support to the merchant. The merchant is able to login as the customer, so it is easier to reproduce the issue of your customer.