Default actions are low-effort actions you take when you are not intending to do anything else. They are almost automatic. Some examples from my life:
Mindlessly check open tabs
Scroll through a news feed
Most default actions are not very productive. However, they are often serendipitous. It’s best to take control and do what you should be doing. Paul Graham said in Hackers and Painters, “If you have a choice between an easy thing and a hard thing, choose the hard thing. Almost always, you’re only considering the easy thing because you know you should be doing the hard thing.”
Make the things you do by default productive and good. For instance, I made a default action of taking my phone out to read ebooks. This is one reason why it's important to set up good default actions. It's about spending your time wisely in non-risky tasks. A related article recommends making high-effort tasks to low-effort.
In conclusion, if you tend to mindlessly check open tabs, open good tabs you want to read and control your tabs, don't let your tabs control you.