Aliases are lovely. They allow you to see what I’ve always termed “friendly names” for field values as opposed to codes and the like. You may have legal entities that are referenced as numbers in the underlying data but you want to display as the names of those entities to the users. You may have state abbreviations that you want to display in full, or a handful of currency codes that you want to show as symbols or full currency names.
I learnt the hard way that creating aliases can be dangerous. Originally, I had no idea that when blending, Tableau uses the alias values as opposed to the raw values coming in on the data feeds. And I learnt that the hard way.
Let’s say my primary data source contains currency codes (EUR, GBP, USD etc.) and this is blended to a secondary data source that contains FX rates that also use those codes. If I add aliases ($, £, €) to the currency codes in the primary data source, then my blend breaks. (I could fix it by applying the same aliases to the secondary data source. But this is not always efficient or wanted.)
To me, this is undesired behaviour. I see aliases as being purely presentational. If I am using a field in two data sources to blend, I want Tableau to respect the raw value rather than some name I’ve chosen to to display to the user.
There is, however, a simple way round the problem: Duplicate your field.
Use the duplicate field to blend on, and apply your aliases to the original field. That way, the integrity of the original field’s name remains intact, and its aliases can be used in presenting the information to the user.