Since the different instruments are not producing the exact same waveforms perfectly phase aligned, there is not nearly a 6 dB boost when the number of instruments playing the same part is doubled. It’s more complicated than that.
The doubling has two effects: first, there is a volume increase, and as some instruments are quieter than others, this can really help with the overall mix of the orchestra. It also helps with playing in bigger rooms. A modern version of this was that James Brown would have two drummers playing the same part so they could be loud enough without amplification. Or the drummers would take turns playing songs because they had to hit so hard they had to take breaks.
The second reason to double parts is to create a different timbre. A violin section playing in unison sounds very different from a single violin playing the same notes. And the size of the section makes a difference. Chamber strings sound “smaller” than symphonic strings.
I guess there’s a third reason which is if you want to create a note of arbitrary length, you can have a section sustain that note for a very long time assuming the players choose different times to breathe (which pros know to do without being asked). So the section sort of turns into a single instrument that has a different timbre and capabilities compared to a solo instrument.