Israel is experiencing 1 COVID-19 death per day on average in a country of over 9 million people. It’s worked beautifully. But it was developed for the April 2020 variant. Delta didn’t exist when the vaccines were developed. And Delta has mutations in key proteins which change their structure such that antibodies for previous SARS-CoV-2 variants don’t fit as well. Along with that, Delta is vastly more infectious (R0 of 8 - 9.5 vs 2.5 for the April 2020 variant and 1.3 for the original Wuhan virus). Even people who were previously infected are more at risk from the Delta variant for these reasons. About 1 in 6,000 previously infected are getting reinfected with the Delta variant.
Even with all that, the vaccines are still working remarkably well for Israel. This move by the Prime Minister is to boost immunity even higher for those most at risk of serious illness in an attempt to ensure deaths REMAIN near-zero. Is it necessary? Arguable. Is it a problem to do it? Likely not. There’s no known risk associated with a third shot for Pfizer or Moderna.
The polio vaccine requires four shots. Does that make it bad? You know anyone in the US who’s had polio recently? Sometimes more does work better.
There's no known info. There is known info that the 2nd shot of these mRNA vaccines seems to produce more adverse reactions than the 1st for individuals.