I remember when I first encountered a software tester. I had just signed up with a publisher, and they employed testers, and one was assigned to my product. His first report was a total shocker. He found so many problems with my product. I thought some of them were really petty, some indicated he didn't understand the product, and in a very few cases he found real bugs.
And having a great tester on a team pretty much guarantees you won't ship garbage. (It can happen.)
My current contract/job is the second project I've had what I would describe as "real" testers. In the past, we had some people who "did testing", but we didn't work that closely with them. Some were in other cities, some were doing more high level testing, and some where just a bit disconnected in general.
As an aside, that's why I don't think outsourced testing works, regardless of if they are in another country, another state, or just another floor of the building. Sure, you can get someone to run thru a script every night, but you can also get a bot to run thru your app every night, too, and bots are cheaper in the long run (eg use Xamarin Test Cloud)
But what a difference it makes having testers who are literally on your team. They are a second set of eyes, a representative of the user and the product owner, and most importantly, NOT close to the code.
The last thing I want to do is throw things over the wall at them, because getting a load of bug reports from a person you interact with all day is horrible - it feels like I'm wasting their time - which I am.
I can highly recommend getting testers on your team, especailly for customer-facing things like apps or websites. There is only so much you can do with automated testing (which is also important) - a human using your product will always pick up different things to a computer making sure the flow works.
Worst case, if you can't get testers, follow this one simple rule:
Never, ever, test your own software.
And always try to keep the testers happy. You're all on the same team after all, and chocolate is cheap.