Well, that content has to take some form. Previously it was all about skeuromorphic design, to make things look like their real-life equivalent.

Then, the balance swung to the other extreme, with completely-flat design. Things were decidedly 2D and "digital".

Material Design is Google's attempt (among others [1]) to standardize while providing subtle cues of depth and interaction.

[1] - https://www.lightningdesignsystem.com/