Migration from SQL Server 2012 Ent to SQL Server 2014 STD
The scripts go over all of your tables, rebuild all the indexes and update their statistics.
The reasons they CAN improve performance is that they reduce the index fragmentation and force SQL Server to generate new execution plans to queries, that may be more suitable.

I'm not sure, though, why you need to run the first command which disables the indexes..

Before running it in production you need to verify they run well on a test system that is similar to the production system. Also do it during a maintenance window and not during the work day.