I was told that when you buy an iPhone/iPad from Apple, as soon as you put a sim in then the phone locks to that network and you need to unlock it to use a different sim.
You were told wrong.
iPhones are sold unlocked, and have been sold unlocked for a number of years now.... you can put whatever SIM you like in them.
If you buy from a phone company, on a contract, then it is a different case, for obvious reasons..... and you will have signed a contract to that effect !
I always thought that jail breaking a phone was for those who wanted to avoid paying top whack for an app via a torrent site.
There is that, and there's also the desire to fiddle with the software on the phone.
But whichever way, jailbreaking is bad because it removes an important security layer.
On an iPhone, apps run in a sandbox, which means they cannot interfere with each other, from Apple's own description :
The security infrastructure in iOS is there to protect your app’s data and the system as a whole. Security breaches can and will happen, so the first line of defense in iOS is to minimize the damage caused by such breaches by securing each app separately in its own sandbox. For security reasons, iOS places each app (including its preferences and data) in a sandbox at install time. A sandbox is a set of fine-grained controls that limit the app’s access to files, preferences, network resources, hardware, and so on. As part of the sandboxing process, the system installs each app in its own sandbox directory, which acts as the home for the app and its data.
Basically, sandboxing prevents a compromised (or dodgy) app from compromising other apps or the rest of the system. Thus it is an critical security feature.