Please note that the information provided here may very well not be 100% correct, and is based on my own (online) research and understandings. I'm absolutely terrible at properly and thoroughly explaining things, so I very much encourage you to correct misinformation you may find, or suggest your own additions/revisions. Thank you! <3
The number of animals inside animal shelters is well in the millions, and although the policies for this vary world-wide, plenty of shelters have to resort to euthanising unwanted pets simply because they lack the room or resources to take proper care of them. Buying rather than adopting a pet further encourages sellers to continue breeding animals, despite there being a surplus of pets to choose from. Be sure to spay and neuter your pets to prevent further unwanted animals from being born!
Additionally, from what I can tell, a lot of American pet stores supposedly get their dogs from so called 'puppy mills' - essentially a commercial breeding facility in which dogs are 'produced' solely for profit, usually in poor living conditions.
By adopting a pet rather than purchasing one, you're potentially saving a life and ensuring your money doesn't go to dodgy breeders - as far as I am aware, animal shelters are largely non-profit organisations ran by volunteers and reliant on outside fundings and adoption fees.
Shelter animal misunderstandings
I've either been unfortunate with the people I've talked to about this, or there's genuinely a bias against shelter animals -- from my experience, people seem to think that animals end up in shelters mainly because they didn't make good pets. This is completely disregarding other possibilities. I've talked to shelter staff, and have been told that more often than not the reason for an animal ending up at them was unfortunate life circumstances of the owners (think moving, poor financial stability, sudden illness - in this particular shelter there was a cat originally owned by a guy that had to go to prison) rather than the pet being a poor companion.
I'm afraid that I personally don't have a lot of direct experience I can add on to this section, as we got our cat as a kitten from a guy who accidentally ended up with a litter - in a way adoption, but she was still young and hadn't been in an actual shelter. However, several friends of mine own cats that came from shelters, and all of them have been nothing but sweet. My favourite out of them is some pitch black cat without eyes; he's the sweetest thing ever and his lack of sight doesn't seem to hinder him an awful lot.
So, as I don't really know what else to add here and am eager to see you guys' input, I'm just going to drop a video of the cat Didga here - . This cat was adopted from a shelter, as well, and as you can see she's still very much awesome.