Hi,

Territory management basically related to sharing model of an organization. We can say it is a structure based sharing. It allow sharing of information according to geographical condition or some specific type of accounts which are categorised by some fields.

Territory management allow us to manage complex sharing between accounts which have some similarity in between. It expands the sharing between same territory Accounts.

There can be several things need to be consider when enabling Territory management:

First thing is it cannot be disabled
It is mostly used where org wide access is set to private and we need to provide some access to specific group of accounts according to their territory. For ex: Forecasting.
Territory affects Contacts, Account, Opportunity, Case and all custom object which have master-detail relationship with account. So sharing to these object should be considered.
If there are some requirement of development on Territory object then it must be known that territory is a Setup Object
By enabling territory all sharing of accounts done according to the territory and their hierarchy.
Forecasting based on territory can be done when enabled.
In summary, territory account organise a complex sharing between accounts of different geographical conditions and saves you from lot of headache of manually setting up sharing for organization having thousands of users and have org wide private sharing for objects. Territory management is a complex and powerful feature to use.

Users are not require for training but Salesforce admin must aware of what he is going to do and what impacts will be on Users using the feature. Also managers like Sales Manager, Forecast manager etc must aware of the change so they can now the new sharing expansion to users in their hierarchy.