Single thread to multi-threaded application

Question

When we should use threads in our application. In other words, when should I convert a single threaded application to multi-threaded application. Being a developer, I think

Answer 1

Reasons I can think of off the top of my head ( and I'm sure there are more ) are :
1. Offloading a batch of work to a worker thread so your program can either continue responding to user input or so you can carry on running other code that doesn't rely on that work.
2. Handling I/O particularly server and network communication where the response time is variable or unknown.
3. Parallel processing of data where you can sub-divide the work down into discrete non-dependant units of work
4. Timer related work i.e 'every 500ms check if x has changed'

However, switching to multi-threaded or concurrent programming is not without it's pitfalls particularly if those threads need to access shared data hence the number of questions on SO about mutexes and thread synchronisation!