Defaults are product decisions
Most users do not actively choose. They accept what is presented to them.
Defaults are decisions made on behalf of the user.And most users accept them.
For eg, when notifications are turned on by default, most users leave them on. When autoplay is enabled, more content gets consumed. When an AI tool pre-fills an answer, most users edit it instead of starting from scratch. When privacy settings are hard to find and change, most people stay with the default settings.
This is because most users are trying to complete a task with the least amount of friction. They rarely stop to optimize settings unless something feels wrong. And that is what makes defaults powerful.
A product can shape behaviour without adding a single new feature, simply by changing what is pre-selected. Spotify deciding to auto-play similar songs after your playlist ends is a default. Gmail categorising your inbox into Primary, Social and Promotions is a default. UPI apps setting one bank account as your primary account is a default. These small decisions quietly shape usage.
When designing defaults, do not just think about what makes the product easier. Also think about what behaviour you want to build.