Vaccines reduce infections considerably.
This is something the CDC has repeated over and over. From the CDC's website [1]:
"In October, the unvaccinated were 5x more likely to become infected."
Not only that - 'breakthrough infections' of the vaccinated are more likely to be less severe, and infection severity is correlated with shedding, meaning, even the 'vaxxed and infected' are less likely to spread.
Vaccines do not stop transmission, but they definitely reduce transmission and have a very material effect on R0.
In fact, by far the largest limiting factor on R0 are vaccines. Far more so than masks, for example.
If you think about it from a systematic perspective, the advantage is reducing transmission is the 'big advantage'.
If a giant wave of COVID wallops through the country and everyone gets it, a lot of people will die even those that are vaxxed. 'Flattening the Curve' is a very real thing. If you watch the % hospital beds occupied, it gets scary sometimes, in some places.
By reducing R0 - especially keeping it below 1 (i.e. not growing in the community) - means you reduce the likelihood that people will get it in the first place (and not take up hospital beds etc..).
Omicron has a bit different characteristics, but it's much the same story.
[1] https://covid.cdc.gov/covid-data-tracker/#rates-by-vaccine-s...