When you click on any of your individual Holdings, the graph and the metrics that are displayed are in the Holding Currency: if you own a house in the UK for example, you will see by default the performance in GBP, regardless of your choice for Owner Currency, which is used only to show you the current value (which is also shown in GBP just above the graph).
Seeing Total Gain, MWR and TWR in the Holding Currency allows you to understand the intrinsic performance of the Investment.
If you switch to Owner Currency view, the return metrics will change (unless Owner and Holding Currency are pegged one to the other, i.e. their FX rate is roughly constant over time), and they will include both intrinsic performance and currency performance.
If the house you bought in the UK a year ago for 100,000 GBP is now worth 110,000 then the MWR is equal to 10%. But if 100,000 GBP were worth 100,000 USD (your Owner Currency) last year and 105,000 USD now, then the current value of your house is 115,500 USD, implying an MWR of 15.5%.
In other words, the return from the intrinsic performance of the investment has been positively impacted by an advantageous currency movement.