it’s not really about saving memory in our case. for small indies I`d say it’s more often about saving the most valuable resource: time. having a modular set means it’s easier to rearrange and create more content quickly. relying on a grid system helps placing objects faster and having a reliable way of making sure to avoid map errors like gaps and z fighting issues. as mentioned above texture space is another factor in this equation. draw calls when handled correctly (mostly by keeping the per object drawcall count low) are not that big a deal. If you have an interior set like alien isolation has for example you have so much occluding geometry and limitede visablity that you can easily get away with 2000 single rendered objects with 5-10 drawcalls each).
a very simple example would be a semi-clean floor in a 12x12 meters room with 9 floor tiles, each 4x4 meters. all of these floor tiles have some different details within (without the details reaching to the borders of the tile so that the illusion of a seamless flat floor is created inbetween the plates - instead of accentuating the edges resulting in a checker board look). you could instead just create a 12x12 tile with multiple textures and export it as that to get rid of the lighting issues but that means that any variation would have to be saved uniquely as well instead of just rearranging the floor tiles. if you multiply that problem over an entire game it becomes an issue for a small team