Because many of my users who use this feed, update their website multiple times per day. So if your feed uploads on the 1st Jan at 1am, and they add a product at 2am that day, and you have it set to once every 30 days, it will be 31 or 32 days until that product is searchable, missing out on potential sales.

30 days for the next feed
+
1 or 2 days for the search to be organised by Google and published.

Personally, if Google published the feed instantly as it was uploaded, i would recommend uploading a feed once or more per day. But at the moment at least they dont, so it is better to do it as above