The aim when shutting down for maintenance (or cold backup) is that the database is left in a consistent state with no need for rollback/recovery on startup.

There are 3 SQL*Plus `shutdown` commands that achieve this in theory, all of which immediately prevent *new* sessions connecting to the instance:

 0. [`shutdown normal`](http://docs.oracle.com/cd/E11882_01/server.112/e25494/start003.htm#ADMIN11157) or just `shutdown`: waits for all sessions to disconnect. This mode is rarely used in practice because it relies on well-behaved clients not leaving connections open. This used to be the only `shutdown` mode that did not cancel running transactions.
 0. [`shutdown transactional`](http://docs.oracle.com/cd/E11882_01/server.112/e25494/start003.htm#ADMIN11159): disconnects sessions once currently running transactions complete, preventing new transactions from beginning.
 0. [`shutdown immediate`](http://docs.oracle.com/cd/E11882_01/server.112/e25494/start003.htm#ADMIN11158): disconnects all sessions immedately and rolls back interrupted transactions before shutting down. Note that the disconnections are immediate, but *the shutdown may not be* as any interrupted transactions may take time to roll back.

The fourth mode of `shutdown` is [`shutdown abort`](http://docs.oracle.com/cd/E11882_01/server.112/e25494/start003.htm#ADMIN11160). This is like pulling the power cord - the instance stops *now* without any cleanup. You usually want to bring the database up again afterwards and shut down cleanly immediately afterwards as in your example. The concepts guide [says](http://docs.oracle.com/cd/E11882_01/server.112/e25789/startup.htm#CNCPT89041):

> This mode is intended for emergency situations, such as when no other form of shutdown is successful.

All the examples you give [perform a checkpoint](http://docs.oracle.com/cd/E11882_01/server.112/e25789/startup.htm#CNCPT89041) as part of the `shutdown [normal]` or `shutdown immediate` so explicit checkpointing is presumably to [reduce the time required for recovery](http://docs.oracle.com/cd/E11882_01/server.112/e25789/startup.htm#CNCPT89043).

general advice:

 * Do not use `shutdown normal`.
 * Use `shutdown transactional` *for attended shutdown only*, when you want to minimise cancelled transactions (attended only because this kind of shutdown [is not guaranteed to shut the database down](http://docs.oracle.com/cd/E11882_01/server.112/e25494/start003.htm#ADMIN11161) at all if timeouts are breached).
 * Use `shutdown immediate` for unattended shutdown or when you do not care about currently running transactions.
 * Do not use `shutdown abort` (plus startup/shutdown) unless you have to - this was more common in much earlier versions of Oracle that it is today. In other situations (not patch/upgrade), if you have a need to [minimise downtime](http://www.speakeasy.org/~jwilton/oracle/shutdown-abort-bad.html) this mode may be appropriate.