This is also a much more natural way of thinking about your design, you can think of the design as a whole working system. Sizing parts to fit to other components in your design, and making them fit perfectly first time. So you can design in the context of your overall assembly and design. You can build 1 part or 1 million parts in the same file. In IronCAD, part and assembly files are the same. Parts made in a separate file can often not fit correctly the first time, so you have to spend unnecessary time going back and forth changing these parts just to get them to work the way they should from the start. This creates an overall worse design experience. Designing this way also means in most cases you’re not designing within the context of the assembly, so it’s harder to see how your changes affect the entire model. Even if these parts aren’t moving parts, you’ll likely still need to mate them to lock them down.
If they don’t fit you then make changes and mate and align them again. You also have to go through the tedious process of mating and aligning all these parts to assemble your assembly, fitting them into their correct positions in a completely separate process to when you made them. This means you have to deal with many separate files that can easily be lost or misplaced on a computer and are hard to share. You model all your parts in separate files, then combine them into an assembly at the end with a combination of mates and constraints. In SolidWorks, part and assembly files are different.