In the long run, it's worse to make overstating claims because it undermines trust in the original message. The main criticism of conspiracy theorists (pick you brand) is that they do not follow the evidence trail. If you want to be on "the right side", you need to ensure your claims are better backed up, otherwise its just two sides making stuff up until one person gives up.

As with OP, I support wearing masks and do so all the time if needed (not American though, so it doesn't have the political bent here it does in the states).

If in fact it's still very likely you can get severe Covid while wearing a mask, then sending the message that "wearing a mask will almost certainly prevent severe Covid" will likely lead people to engage in riskier situations with their masks. For instance, it could be that going to a nightclub while wearing a mask is still pretty risky. Telling people that wearing a mask eliminates risk would likely send more mask-wearing people to said nightclubs.

Of course, if wearing a mask was in fact extremely effective at stopping severe Covid, then one could make the argument that it's now worth doing these higher risk activities.

Intellectual honesty?

Not giving more ammunition to the anti-mask crowd by knowingly spreading misinformation?

because wearing mask can be uncomfortable, why wear if the benefit is only minimal

I’m not sure if it’s just as critical or actually more critical to make sure “your side” is not disseminating fake news.

The truth can be valuable in ways that aren't immediately evident.

For instance, if evidence emerges that masks have some harmful effect, an over-estimation of the protective effect of masks can lead to a skewed cost-benefit analysis that results in a net-harmful pro-mask policy remaining in place.

Lies can also undermine credibility, and sometimes necessitate more lies to cover up for the first, leading to less effective and efficient communication amongst people.