Our main motivator was, and is, that manual testing is very time consuming and uninteresting for devs. Spending upwards of a week before a release because the teams has to setup, pick and perform all featue tests again on the release candidate, is both time and money. And we saw things slip through now and then.

Our application is time critical, legacy code of about 30 years, spread between C# and database code, running in different variations with different requirements. So a single page may show differently depending on where it's running. Changing one thing can often affect others, so for us it is sometimes very tiresome to verify even the smalles changes since they may affect different variants. Since there is no automated tests, especially for GUI (which we also do not unit test much, because that is complicated and prone to breaking), we have to not only test changes, but often check for regression by comparing by hand to the old version.

We have a complicated system with a few intergrations, setting up all test scenarios not only takes time during testing, but also time for the dev to prepare the instructions for. And I mentioned calculations, going through all motions to verify that a calculated result is the same between two version is a awfully boring experience when that is exaclty something automated tests can just completely take over for you.

As our application is projected to grow, so does all of this manual testing required for a single change. So putting all that effort into manual testing and preparation can intsead often just be put on making tests that check requirements. And once our coverage is good enough, we can only manuall test interfaces, and leave a lot of the complicated edge cases and calculcation tests to automated tests. It's a bit idealistic to say automated tests can do everything, but they can certainly remove the most boring parts.