Adding to @bcioutier's great answer, it is somewhat wrong to assume there are counters only in Japanese or there are no counters in English as @Eiríkr Útlendi commented. I am not saying you are assuming so.
English uses a couple to indicate two. And there are a half a dozen to mean six, a dozen for twelve, a score for 20, a gross (12 dozens) for 144, etc.
There are also special words for people using prefixes for each number.
Soloist, duet, trio, quartet, quintet, sextet, septet, octet.
Why are counters (助数詞) necessary to describe the numerical quantity of
objects in Japanese?
Japanese needs counters to describe quantity of objects, some are complicated, some are not. But that's the way the language has evolved for thousands of years as English has on its own way. If someone asks why English uses duet and trio instead of just 'two people' and 'three people', you will answer because they have specific meaning to indicate the number of people in music or entertainment.
how strict those who are fluent tend to be about it in colloquial
It will depend on each native speaker. There are some counters which are strictly followed and some are not.
For more information, you can read the link on Words for numbers in English and Japanese counter word.