Many keeper use these but as far as I'm aware there's little real evidence they are beneficial even though many people believe so. Interesting most people who use them cite different reasons and benefits for their use. The usual get out clause for alternative medicine is to claim it to be a preventative or repellant, if parasites are involved. They never claim to be "a cure for". The usual expression is "helps with" or "beneficial in the treatment of", and if you see these weasel words you can be pretty sure they will be useless other than as a crutch.
ACV's widespread use probably dates from a seminal book on alternative medicine "Folk Medicine: A Vermont Doctor’s Guide to Good Health", written by D.C. Jarvis, M.D., which was published in 1958. It became a cult best seller. ACV has been claimed to offer all sorts of benefits (as well as some problems). It has been claimed as a treatment for diabetes, weight loss, cholesterol reduction, and dandruff to name a few of the more popular ones. Virtually all the benefits claimed are based on unsubstanciated theory, for example the supposed benefits of body acid balance, some of which is known to be false (including those in Jarvis' book.).
Some people believe ACV contains benefitial tannins which help digestion. Though in fact any red skinned fruit such as apple will contain more tannin. What I will say about ACV is that chickens seem to like it and anything which causes them to drink plenty of water can't be too bad. I sometimes use it for about a week continuously if I think they need more hydration or after a moult. The type you need is the non-refined type sold by horse feed stuff stores not the pasteurised and filtered stuff you buy at Sainsburys.
Garlic is generally believed to be a powerful medicine by herbalists. It is a broad spectrum antibiotic which it is claimed can be rubbed on a wound to prevent infection. I've no idea if digesting it has the same beneficial effects and you shouldn't assume so without clinical proof. Garlic can also cause blistering and an allergic reaction so I don't advise using it on wounds.
Substances which are effective on bacteria on a lab slide maybe completely useless when taken internally. For instance Fairy Liquid will kill cancer cells in a test tube but it's no use as a cure for cancer so you certainly shouldn't drink it. Again garlic is an appetiser and it might make food or other medication more palatable. I don't use garlic though I can't see it is likely do much harm.
Garlic is also claimed to reduce cholesterol, help with weight loss (sound familiar?) and is an antioxidant, though the elimination of free radicals in the body is losing credibility as being beneficial for cancer and aging. On the down side raw garlic's high sulphur level (which gives it it's antibiotic properties) is an ideal environment for botulism which causes a dangerous digestion toxin.
Although I don't underestimate the placebo effect of using alternative medicines generally I don't use supplements unless they are part of a medical treatment or are given for some very specific reason. If you use any supplement I suggest you do so with a specific reason in mind and some expected outcome. Just using them speculatively on a regular basis doesn't teach you anything about their effectiveness or value when you really might need them. The whole subject of nutrition and supplements is riddled with quackery and inflated claims for health giving panaceas so don't be taken in.
HF