There are various reliable sites and information about the benefits of garlic on the internet. This is one statement:
Antibiotic properties - Garlic contains allicin, which acts as an antibiotic. Garlic releases allicin when a person chops or crushes it.
You might want more of an explanation to your question of "what is garlic supplement good for" and googling it in different ways will bring much good information up for you.

I have understood, hopefully I understand correctly, that garlic has several benefits for us.
I am one of those who uses garlic a great deal in my cooking, besides taking a garlic supplement.
So far with all that I do take as supplements they have not had a negative effect on me and I've been a believer in taking supplements in moderation as long as you understand, somewhat, the body chemistry and its needs .....and take only what you need. For me with BE/MAI I take supplements and eat foods that I feel will keep my immune system strong.

This BE/MAI health problem flared up during a period of high stress and during that period of unrelenting stress I, as we all were, was learning how to deal with the concerns of Covid, the vaccine decision etc etc...... Foolishly during that period I was not doing right by myself and made some not so beneficial choices that caused a weakened immune system.

When doing right by myself and paying close attention to my physical being and the little bit I have learned along the way my body has healed and come back from a couple of serious illnesses.
As I have mentioned in another post, luckily, this is my first chronic illness which started showing it's 'ugly face' at age 78, diagnosed at age 80. I was late in learning much about what one needs to do for oneself with BE/MAC and have been greatly helped by all on this site.

I will never fully know what one thing I am doing that might be helping me keep my infection down (I have not taken antibiotics as yet) however I have understood for years that garlic is a good source for several things we concern ourselves with in trying to stay as healthy as we can.....however....things do happen, hello BE/MAI. I consider myself fortunate in many ways and am so thankful for that. My one good fortune is that I am able to easily bring up and out the mucus and mucus plugs of BE. For me water/moisture intake is one big key for getting it up and out.
I am hoping my C Scans continue to show a lessening of inflammation and hopefully improvement in all areas of the lungs. Fingers crossed.
Hoping for all good results for you as well.
Barbara