For me, there are three main reasons to build modules instead a home layout: Space, flexibility and they are achievable.
Modules take up less space
It’s not that simple, but at least modules may take up less space in your home, than a static layout. At least, until you built enough modules to fill your basement. I built about 20 m worth of modules, but I don’t have a dedicated room for my model railroad. I can store most of my modules below the stairs into the basement. These modules were part of modular N-Scale layouts that were larger than my house. When I want to work on a module, the module can sit on workbench, others use their kitchen table. So if space is an issue, modules may be a solution to have a layout – or parts of it – and have fun on large layouts.
Modules are flexible
There are two aspects to this: The layout itself and the focus of your modelling interest.
If you build a home layout, you are commited to a trackplan and a topic. If you are like me, you might loose interest into the selected field of interest and might want to model something different. A module can be put away and make room for something new. That would be cumbersome with a basement filling layout.
A home layout is static – unless its modular – thus operations on your layout are … well … less varied. At some point in time, you will have switched all spots and run all trains. We never build the same modular layout twice. Every layout is different, yes the modules might be the same, but how they are arranged and operated is planned new for every setup. I personally could never build a layout at home, that would be that large or that interesting.
If you like this flexibility, building modules and joining a club might be a thing for you.
A Module can be finished
You can take your module to a meeting, as soon there track on it and the electrics are working. And it’s a good idea to test it before ballasting the track. Depending on the size, a module can be built in a few days or may even take a multiple years complete. You can pause working on a module and work on something else. Usually working on a module is not as overwhelming, as having to fill the whole basement.
Yes, building the layout is a major contribution to the enjoyment of the hobby. Completing something is also quite satisfying. Researching, planing and building something new is also very interesting. And yes, operations are fine too.
There is even more
I consider these factors as significant, but there is more. Attending a modular meeting is a lot of work and it even is exhausting. At least for me, making friends and having fun with them is even more important than working on the modules itself.