In addition to what Machu noted regarding preventing infection, there is widespread speculation (and possibly some emerging data to support it, I just haven't had a chance to look it up recently) that it reduces viral load which reduces the possibility of spreading it. The CDC has relaxed 'mask guidance' in some situations due to this concept. But, as to the current spikes, I think it's accurate to assume that infections will spike but that deaths and hospitalizations may not since the vulnerable have had some opportunity at this point to receive the vaccine. Purely my own speculation, but I think the current spikes are a factor of winter letting up and populations becoming more mobile as a result, along with some fairly widespread 'letting your guard down,' as you noted. Plus, the populations who have never taken this seriously still not taking this seriously.