Is removing an app any different from disabling an app? One of the main advantage of rooting is removing bloatware [I guess]. Is removing unwanted brand-based or operator-based app any different from disabling it from All Application in Settings? I don't find any improvement in memory too since these apps were pre-loaded in OS and you can't use those space any way.
As it has already been said: the main difference is that disabling an app simply marks it unavailable (which can easily be reverted), while removing physically removes the app and all connected data from the device.

What was still left open is: What do you gain from removing an app, that you didn't get by disabling it – so it would be worth the risk of not being easily reverted?


Space: You're probably talking about system apps here. Those are usually installed on the /system partition, which is mounted read-only, and cannot be used by "normal (user) applications". So removing the app itself doesn't give you space to "simply install other apps". But still:


Almost all apps have data, which is stored in /data/data/&lt;app_package_name&gt;1. While pressing "clear cache" and "delete data" from Settings→Apps resets those, it doesn't completely delete everything. Removing the app does. So you gain at least some space here.
All apps are being "optimized", which results in Dalvik/ART cache data residing on the user partition (inside /data/dalvik-cache). If you delete an app, this cache is removed – if you disable it, it remains (see the comment of Death Mask Salesman below).

Performance &amp; battery: If you disable an app, it becomes invisible to the user – but the files are still there. Also, the package manager still knows it exists. Plus the app's "intents" are still available: if another apps directly calls them, the disabled app still responds to the call. It also seems to listen to some broadcasts it previously had registered listeners on (example: my LG Optimus 4X has those bloated LG stuff I've disabled. Nevertheless, after each boot I receive prompts from their "remote service" app to approve).
That means, a disabled app can still be running in background, if called from some other place – and consume battery plus CPU, even bandwidth and other resources. A removed app obviously can't.




1 Some apps also store (usually "huger chunks" of) data on SDCard, but almost all apps have at least their central data below /data/data.