Greedo my 2.2 cents worth (GST inc)
We use 6000FT at YBBN to allow for auto releases. INbound acft assigned 7000FT then managed vertically by approach below that.
Its called separation assurance. We used to have aircraft cleared through to FL200 and had a few nasties. The 6000FT serves well.
YBCG has a cap of 6000FT for different traffic management reasons as well as catering for the route lowest safe to the south of 5700FT.
Like wise YSSY etc.