There are two main reasons as to why you would want to proxy the WMTS basemap, these are:
You may have to provide a customer API Key in the URL for each request, as in the case with the OS Maps API. If this were to be used directly in iShare, the key would have to be transferred to the client which could expose it to misuse.
Proxying the basemap through MapProxy allows us to also query it directly with WMS which then means that it should work seamlessly with printing.
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