Might be better stated as "altruistic acts", but the primary reason is that the benefactor's anonymity preserves the dignity of the recipient. When done privately:

1. The recipient doesn't have to cop to/explain their situation that prompted the generosity,

2. The recipient doesn't feel the need to publicly acknowledge the help (which in turn implies publicly explaining that you needed help),

3. The recipient doesn't become a revenue-generator without giving their permission, as happens in this case and others like it

When done in public, these acts cease to be altruism - they're now a business exchange ("content creator" gives you something of value (flowers) in exchange for something else of value (clicks/views/clout)). Except you didn't know you were making this business deal, which is beyond slimy.

IIRC most religious texts teach to do altruism privately, for reasons such as this. Attention-seeking didn't just come about in the internet age, it just got easier.

I would alter that slightly: virtuous acts should be done whenever an opportunity arises. When we see particularly conspicuous acts of virtue, by which the practitioner benefits greatly, especially when the opportunity to avoid conspicuousness existed but was eschewed, we are right to suspect that the act would not have taken place in the absence of an audience, and thus actually signals a lack of virtue. (Because the virtuous act so regardless of audience, and personal benefit, and even at great personal risk or hardship).