There may be a couple of reasons for the Y/Z approaches.
If you look at the reference beacon for the ILS's you've given in your example, the ILS-Y uses the SY DME, and the ILS-Z uses the ILS DME. SO, you'd be foolish to use the ILS-Y if the SY DME was notamed U/S.
If you look at the CBR 35 ILS Y and Z the reference DME is the same. BUT the CBR 35 ILS-Y allows you to have a DA of 2070', much lower than the CBR 35 ILS-Z, which only allows you down to 2100'. The one you use in this case will be dependent on what missed approach climb gradient you aircraft is capable of.
There may be other reasons, but they're the two that I've had experience.
Cheers,
DIVOSH!