Even small changes might have ripple effects over the product. Allegedly, Walmart lost $100M within 10 minutes because of a bug in production. It happens quite rarely, but you don't want it to happen. It is like insurance. If your coverage is 40% or less - do you trust it? Why can't you just write 100% code coverage and be done with it? - well because it is almost impossible. Almost nobody (except you are on a major project in a major bank) have enough resources to build enough tests.
Diminishing Returns :(
Unfortunately, with any code, Selenium included the more code you have the more work it takes to support it. Literally. Up to a point where the whole development of new test is reduced to a crawl. Just because developers just updated their HTML structure yet again.
Speed of change is critical. It helps to move organization forward faster. What if the change is a BLOCKER level security issue and needs to be deployed ASAP? If you don't have enough test coverage - then you just hope (not test) that you didn't miss anything important. If your test coverage is 30% and everything else is supposed to be tested manually - what do you do?