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It's to maintain the required minimum flow-rate to protect the boiler and pump on overrun before it switches itself out. (i.e. with full TRVs it is entirely possible for all the radiators to be in a shut state. The boiler doesn't receive sufficient flow to dissipate heat properly, and the pump can be working against full resistance. One "always on" radiator avoids that situation. A similar (though slightly different) method is to fit a bypass-valve, which allows a permanent (or automatically pressure controlled) cross-bleed between the feed and return circuits.
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