This could be a pretty long post so bear with me. Diseases and parasites can spread many different ways: from sharing food or water dishes, from eating each other's poop, just by being next to each other, through certain vectors like mosquitoes grubs grasshoppers or other insects, or even being carried on the air. It depends on what the disease or parasite is and whether or not they are infected to start with.
It is fairly common for a flock to develop immunity against a certain disease or parasite. Although they are infected and can pass it on to other chickens they are immune to its effects. Coccidiosis is a good example but there are several others. This could just as easily be the flock you already have as the newcomers bringing it in.
If the birds are coming from a stable place where they are not meeting new birds all the time then quarantine is not likely to do you much good. They have basically been through a quarantine. That's not your situation though, quarantine is pretty effective for birds coming from an auction or a swap where they have been exposed to strange animals. A lot of people do like you had been doing, just bring them home and turn them loose, usually without suffering any bad consequences. The most likely consequences are mites, lice, worms, or coccidiosis. These are unpleasant but if you pay attention and deal with them you are not likely to lose your entire flock. It's not a bad idea to treat the new ones for mites, lice, and worms while they are in quarantine just as a precaution.
But it is also possible you can bring in some disease that will wipe out your flock. This is the real purpose of quarantine. Usually you keep them in quarantine for about 30 days. Most things should show up by then.
Because different diseases and parasites can possibly be transmitted by so many different ways it is hard to get a real effective quarantine. Most of us are not set up where we can keep wind from blowing from one area to another or keep mosquitoes out. The better you can isolate the two groups the better. There are different degrees of quarantine. Even if you can't do that you can help stop other things from spreading. There is some benefit from a lower level of quarantine.
You could easily be the vector that transmits a disease or parasite from one group to another. Don't store their food together and don't use the same buckets to carry food or water. Change shoes between groups so you don't carry something in them. Some people go so far as to change clothes. How far you want to carry quarantine procedure depends on you and your set-up.
One way to handle flock immunities where the problem is highly unlikely to show up no matter how long you quarantine them is to pick a potential sacrifice from your existing flock and house it with the new ones. If the new ones have problems your existing flock is the carrier. If the sacrifice has problems then it's the new ones. If both have problems then you hit the jackpot.
What do you look for in quarantine other than basic parasites? A chicken acting odd. Most of the time if a chicken is bad sick it will stand around fluffed up, hunched up, and looking lethargic. Observe their poop. There is a lot of different variety in what a healthy chicken will pass but there are charts online that show abnormal poop too.
The degree of quarantine you decide (if any) will depend on how hard you want to work, how you can set it up, and how risk adverse you are with your current flock. Some people consider their birds as pets and almost family members. With others they are more livestock and are more willing to take a risk. But people that keep livestock generally know that healthy birds are productive birds so don't like to bring in diseases or parasites either. After all, they are the ones that developed quarantine procedures.