What is UUId and Id?

Local ID
Number value by which an Appian object (such as documents, knowledge centers, groups, and users) is referenced on an Appian server. Unlike UUIDs, the local ID may change from one Appian server to another.

Universally Unique Identifier (UUID)
String of 32 hexadecimal digits assigned to an Appian object when it is created. This internal (hidden) property is not editable or configurable. When migrating, UUIDs are also assigned to existing objects. UUIDs are used when importing an application to determine whether or not (regardless of the object's name or Local ID) a certain Appian object already exists. If an existing object is found, the import process updates the existing object, rather than creating a new one on the target system.

Every appian artifact has a Loca Id and UUID.
Lets say you have a constant in the Environment DEV and has
Id:131
UUID:_a-0000dee4-8f57-8000-3457-01ef9001ef90_2806

So if you import this object to other environment UAT then the ID might change but the UUID will be the same as above

Id:2456
UUID:_a-0000dee4-8f57-8000-3457-01ef9001ef90_2806

Like wise the UUID will be unique across all the environments and Id might change from one environment to other environment.

Hope you understand the differences.

Here I have a doubt: 

Id:131
UUID:_a-0000dee4-8f57-8000-3457-01ef9001ef90_2806

So if you import this object to other environments UAT then the ID might change but the UUID will be the same as above

When Id changes then we can use only UUID right? My question is why do we need both ID and UUID.

Thank you for the help in advance.

In your daily live you do not need any of the IDs. When you reference an object you use a constant which internally uses the UUID.

Documents uploaded by users are referenced by their IDs but will not be part of a deployment package.