It's mostly a technical reason, eg: eCommerce requires Website and Sale because you need a website to run a shop, and Sale to manage orders.
Note that for the free for one app offer, you get the app (eCommerce) as well as it's dependencies (website & sale) as part of the free plan.
Over time, we try to remove dependencies (ex: in v10, eSign will not depend on website anymore. Same for mass mailing) but some will always remain because of the string integration of the apps in Odoo.
You can easily display all dependencies :
display apps in view list,
select some or all of them
on the left button that will be shown, choose Print / Technical Guide
Now you have a big PDF with all dependencies (and a lot more informations).
Another way it to browse the Odoo install directory under "addons" and to extract all dependencies from "__openerp__.py" files (each module (or addon or App) contains at least one file with a depend list. Like for example :
About This Community
|Asked: 9/10/16, 12:38 PM|
|Seen: 175 times|
|Last updated: 9/20/16, 6:37 PM|