• Because we’ve heard countles of stories of people that forgot to deactivate the pass after they bought it and activated first. When they start to travel, it appears that their pass is already not valid any more. Or you encountered a problem early on their presumed first travel day. This can be delay, strike, serious defect on the line making rail traffic difficult for the rest of the day, illness, emergency or some other issue that can thwarth your plans for that day, forcing you to replan or postpone travel. That way you can lose losing some travel days of your pass.
• There’s a difference between planning and adding trip to your plans. With the first one there’s nothing wrong, most people plan in advance.
  What we warn for is that you shouldn’t activate a travel day (tapping the yellow ledger to add a jourey to your pass and to generate a qr-code) in advance because of the reasons in the first paragraph. Something unexpected can quickly happen. Just wait to activate a travel day until you see your first train of the day, standing at the platform. Since you cannot get travel days back, once the day started.
• You can be certain, but there’s always a chance that you get ill, there can be uneforseen circumstances like closed railway lines like the Gotthard, Modane-Turin, some routes in Norway because of storm damage,… at the moment, a strike,...) Not that this happens a lot, but the process of getting a travel day back that you didn’t use is cumbersome, and not sure. And can get you in trouble for the remainder of your travels.

Just by waiting to activate your pass on your first travel day and activating travel days only minutes before taking your first train of the day. You just avoid getting in potential trouble by unforeseen circumstances.

Wow, thanks for the quick and detailed answer! :D