"Is pickup necessary in the first place?"
Originally, acoustic guitars and gut guitars are instruments that take the vibration of strings from the sound hole into the body, and make the body's raw wood resonate and enjoy its sounds.
There is no problem even without a pickup.
"So why is it necessary?"
When a big sound is needed at the shows etc. held in a large halls, when you want to play and need acoustic guitar sound in a band, when you want to produce and enjoy the same sound of favorite artist....etc., there are many reasons depends on how to use and enjoy.
Historically, it was for improve convenience in the band sound.
An acoustic guitar used in conjunction with electric instruments inevitably lose volume. So it is necessary to be collecting sound by standing of a microphone, but the microphone also picks up sounds other than the guitar because of high performance. So knowledge of the microphone is required such as setting method and type.
The biggest disadvantage of using a microphone is the annoying howling which is caused by sound looping, and that you cannot move from place of standing a microphone while you are playing the guitar.
Pickups were developed in order to isolate even a little from howling trouble for picking up only the sound of the desired guitar and amplify. It was a very simple purpose.
The under saddle piezo pickup that picking up only the vibration of the string by setting under the saddle, the magnetic pickup utilizing the magnetic vibrations of the string vibration with the same principle as the electric guitar by setting in the sound hall, such as body contact piezo that represents the BARCUS BERRY that became popular in the 1970s to 1980s etc., many various pickups came out.
When I was young, I imitated well by looking at Japanese folk song musician who attached pickups to the end of bridge by clay and put cable by tape to guitar, because especially BARCUS BERRY etc. didn't have to modify guitar. However after that, the traces of tape were left on guitar is one of my bitter experience. Anyway in those days, the sound was bad but it was a breakthrough that sound of acoustic guitar strings came hear from in a band.
Since then, year after year, it has been improved by many people and many sophisticated products are on the market.
And recently, though people who personally record at home and upload to SNS site are increased, it is convenient because of be able to send only sound signal of desired guitar to the computer by line directly just connect to the device if your guitar set the pickup.
When you ask someone for setting a pickup on your guitar, I recommend that you talk about how to use and enjoy your guitar, and also what kind of sounds you like etc., to the craftsman.
I always keep in mind to set up a pickup that meet such user needs as much as possible.
I think there are such craftsmen in your country too, so please try to look for and talk to them.