Alberto Ferrari explains the LOOKUPVALUE function:
LOOKUPVALUE is one of the most widely used functions, especially for DAX developers who come from an Excel background. Indeed, the behavior of LOOKUPVALUE is very close to the behavior of the widely-adopted VLOOKUP function in Excel. Yet, there are important differences between the two; quite often, newbies use LOOKUPVALUE instead of creating a relationship between tables that ensures higher flexibility and higher performance.
This article is an introduction to LOOKUPVALUE. If you are interested in studying the internals of LOOKUPVALUE in more detail, make sure to check the advanced LOOKUPVALUE article; in that article, we describe the behavior of the function in detail along with several performance considerations.
Click through for the article.