Modules Composer is not an editor or a builder. Is just a Composer in which you select the modules of your choice and then you can export them in your desktop and use them on MailChimp for further customization.
The main reason that Modules Composer exists is just that MailChimp cannot handle all these 100 or 200 modules. If you try to use on MailChimp more than 30 modules it will go extremely slow and it will crash. So here is where Modules Composer can be helpful. You only choose the modules of your choice and then you can customize them on MailChimp much faster and with no crash experiences.
Here is a video which shows how to use Modules Composer – https://youtu.be/zyik_nxS2W0 and here is a video which shows how to upload, edit and customize our email templates on MailChimp – https://youtu.be/Itmu3iA6GFQ
At your disposal,