Hi, I would say always, or almost always. The 3 tier architecture is very common and helps to make developments easier and faster.
First tier holds qvds with flat information from db, this helps in analyzing source data and free resorces from db server. You extract the information, create the qvd and next queries will be on qvd, not to db server.
Second tier is transforming this flat data from tier 1 in the bussiness data needed by your apps, this can be done in one or many layer of qvds, reading from tier 1 qvds and creating tier 2 qvds, with all the fields needed by your apps.
Third tier is the presentation layer, the apps the user will use with charts, tables... with almost no script, just doing an optimized load of tier 2 qvds. In some cases doing a binary load of data model apps.
https://community.qlik.com/t5/QlikView-Documents/Three-Tier-Architecture/ta-p/1483564
