You probably won't know unless you ask the design team, but there are some principles for hierarchy and salience.
Without having access to the decision making process, this is speculative.
However, there are some popular guidelines in use for determining the perceived importance of a UI element.
Google Material as an example
Buttons take on 3 levels of salience:
- Text button (low emphasis)
Text buttons are typically used for less important actions.
- Outlined Button (medium emphasis)
Outlined buttons are used for more emphasis than text buttons due to the stroke.
- Contained button (high emphasis)
Contained buttons have more emphasis, as they use a color fill and shadow.
- Toggle button
Toggle buttons group a set of actions using layout and spacing. They’re used less often than other button types.
Generally, when you get to Contained buttons, these are the primary action on the screen, ideally the one thing that's most important to do.
As this applies to you directly, you as a designer need to weigh the actions in the application, their context in a view, and what outcomes are important to the user (and your business model). That can assist in choosing the proper form factor to achieve the outcomes you need.