Why do indexes, statistics, and full text need to be rebuilt/updated/repopulated after migration?

I have created a new server, restored the system dbs and presented a copy of the data/log disks from the previous machine (or alternatively maybe I restore the user dbs using the .bak files). Post SQL Server migration, why do the following need to be performed: 1. Index rebuild on all databases 2. Statistics update on all databases 3. Repopulation of all full text indexes (or rebuild all full text catalogs) Example: https://www.sqlservercentral.com/articles/sql-server-migration-1 Does the answer differ whether the upgrade was done in-place vs by restoring from bak files? And server version vs different version?