Please have a closer look at the documentation as it is very clear about the benefits of KEEP versus JOIN:
- One of the main features of Qlik Sense is its ability to make associations between multiple tables instead of joining them, which greatly reduces memory usage, increases processing speed and offers enormous flexibility. Explicit joins should therefore generally be avoided in Qlik Sense scripts. The keep functionality was designed to reduce the number of cases where explicit joins needs to be used.
https://help.qlik.com/en-US/sense/September2018/Subsystems/Hub/Content/Sense_Hub/Scripting/ScriptPre...
