Are automated tests really that important and worth all the time and costs?

Hi!

I work at a small company. I have been there for 2-3 years so I'm still a junior with not that much experience I guess. There are no strict rules that x% of the code has to be covered by automated tests. In my own projects I haven't written any tests at all. The reason is that writing all the tests would take way too many hours and would add tons of hours and costs before the product is finished. I don't know if I could persuade the customer or our seller that it will take 1-2 weeks extra of full-time work just to write the tests. So I just test everything manually as I go and before shipping the product.

Do you write tests? Why? What is your opinion on automated tests?

Hope you're having a chill weekend.