@ Edinburgh_lad that is not true, on previous models it works without having to issue the savecfg command after, and setting is updated even after reboot. Sorry to sound like an arse but have you even done this on draytek's before?!
There's no need to be arrogant about it.
No-one is asking you to use 'savecfg', so don't feel compelled to use it, you know, if you don't want to or know better.
The savecfg command (if it's there, it's there for a reason) is to ensure that the configuration is saved to the flash memory and that the SNR stays as requested etc.
My line, for example, suffers from some interference, which occurs locally (our pole only). Without the savecfg command, the DSLAM would eventually reduce (and has reduced) my SNR to 3db after a period of relative stability, causing my line then to become less stable. The savecfg command ensures that the SNR stays at 6db until I call for a new SNR change (or do a complete reset of the router).
PS Not Draytek's but Drayteks. The apostrophe is only used for possession and contraction, but rarely for the plural (exception is, for example, CD's, though it's CDs more often these days).