What benefit do fully vaccinated people get?

The benefit is that they don't contract the disease (or don't get it as severely).  If the most vulnerable are vaccinated (the elderly and those with underlying conditions), then the death rate should plummet.  At that point, it will be like the regular flu--killing a few tens of thousands a year, but otherwise not having much of an impact.

Young and healthy people getting Covid isn't really a problem, because they mostly recover (if they even have any symptoms at all).

They benefit from not experiencing severe COVID symptoms or not experiencing symptoms at all if they are infected when they might previously have been hospitalized. But they will still be capable of shedding virus so masks and social distancing will be required until the majority of the population has been protected from severe symptoms by vaccination. COVID will probably become another seasonal disease like flu which is worse in the winter months when people are more likely to be indoors spreading the virus. Like flu it will probably be necessary for the most vulnerable at least to be vaccinated against the variant of the virus expected that winter.

we are not likely to have herd immunity by summer -- and if resistance to the vaccine continues at present levels, may never have it.