The modular design has been a sympathy to many companies that wanted to embrace it on their own devices. In fact, that design allows the users to enhance the functionality on their device by purchasing additional modules that can perform a specific function.
Many of the modules we’ve seen so far, were used for camera enhancing and to grant better sound experience. Samsung have not done it so far on any of their device, and as the new rumor says, they might add it to their upcoming Galaxy S9.
The implementation of the modular design on the S9 might be similar to the one that Motorola Moto Z line has. If it turns out to be true, the Galaxy S9 might have magnetic pins at the back side which will be used to attach external modules easier.
We are still very far away from the Galaxy S9, but as the time comes, we hear about it more and more. The Galaxy S8 is by the way doing phenomenal job, sitting as the most selling device from Samsung ever. Also, the company have not shared any info about the Galaxy S9, but we expect that we will see it in the next year.