1) There are loads of lovely dogs there that need a good home that probably didn't have a good start in life and therefore need to be shown some love and kindness. They'll have a second chance at living a happy, normal life.
2) Do you really know where that dog or cat came from? Many dogs and kittens sold in stores or by individuals online have been cruelly bred in puppy farms or kitten farms, their mothers utterly abused in the process. By getting your pet from a shelter you are choosing to not fund animal cruelty. You're saying no to pets bred at the expense of their mother and exploited, sold off for money, without proper veterinary care or socialisation.
3) Many animal shelters are locally run by people from their own pocket or a small amount of charitable funding. Support your local shelter today.
4) You could adopt and older cat/dog after meeting it which gives you the chance to really assess their personality. Sometimes a kitten or puppy acts completely differently once they've reached adulthood. It also means there will be someone to care for their own independent needs as older cats and dogs may require certain care.
5) Adoption fees are usually lower and cheaper than what you'd pay for a puppy or kitten - especially if they're pedigree. In fact many pedigree dogs end up in shelters later on because people bought them for their aesthetics without considering the responsibilities and realities of pet ownership.
6) Many pets from shelters are already spayed/neutered/micro-chipped before they come to you. They've often had all the appropriate vet checks and injections and so you don't have to pay for them later on.
7) Your home will be more lively with a little cat or dog scampering around.
8) It prevents strays from being put down.
9) You can really suit the situation around your needs. Do you live in an apartment? There are cats that specifically need to be kept indoors. You have young children? The dogs are almost always tested to see if they are compatible to a home with children. So much care and examination goes into the adoption process.
10) You'll know in your heart that you've done something good in the world. It may seem like a small thing, a tiny contribution, but you will totally transform the life of that animal for the better if you can provide it with a loving, supportive home.
Don't overlook a pet because they're older or perhaps disabled. Many cats and dogs in shelters have to deal with missing eyes or legs. This doesn't mean that they have any less love to give, it just means they've endured more before meeting you. My family and I took in a rescue kitten not long ago. She and her siblings had been carelessly dumped in a wheelie bin on a hot day. She's still very much affected by her troubled youth but she has made a wonderful addition to the family. In fact black cats are often less likely to get adopted. Perhaps this is due to superstitions about black cats or maybe people are more in favour of colourful felines. I simply ask that you do not overlook black cats simply due to their colour as it has no impact on their personality or suitability as a pet.