A reason for having 2 ISINs is to use different Central Securities Depositaries (CSDs) for collective safe custody of the securities.
In the above example this would be Clearstream (for DE000A1JXC78) and probably Euroclear (for IE00B5L01S80).
Thus, in this model, if you are located in Germany and buy the DE version then you have securities with a domestic issuance structure whereas with the IE version you have an international issuance structure.
Perhaps for some investors this makes a difference as the domestic issuance structure is perhaps more robust in times of a crisis.
Incidentally, effective 2020-03-23, HSBC switched all their ETFs from the multiple CSD model to an International Central Securities Depositary (ICSD) model. Meaning that now all the DE* ISIN versions are gone and the holt DE* securities are automatically exchanged 1:1 to the corresponding IE* counterparts.
A bit of rationale from the HSBC 2019-11-18 'Change of share settlement structure' announcement:
Trading and settling participating shares
under this [current CSD] structure involves moving the participating shares between various CSDs, which is
complex, costly and inefficient.
The directors of the Company are of the view that the International Central Securities Depositary
(“ICSD”) settlement model provides a more streamlined centralised settlement structure which they
expect will result in improved liquidity and spreads for investors and will reduce risk in the
Converting the settlement model of the shares of the sub-funds of the Company to the ICSD model
will also bring all of the Company’s sub-funds into one consistent settlement structure which is
expected to make navigating the settlement structures for the sub-funds more straightforward.