The line between concurrent and parallel computing is blurry, and it's often debated if they're the same, if one's a subset of another etc. They're generally regarded as separate but related.
Performing multiple tasks at the same time massively speeds up process
Used by GPUs to e.g. render 3D objects quickly
Can have multiple web pages open to improve productivity and enhance usage
Non-trivial to implemen e.g. there's an overhead in co-ordinating multiple processors
Can be unneccesary: processor may not always be performing multiple tasks
Some tasks are even faster on just one proccessor than multiple
Increased throughput, allowing more tasks to be completed in less time
Time that would be otherwise wasted can be spent e.g. instead of waiting for user input, other tasks can be executed in the meantime
Slower than parallel processing
The handling of large calculations or multiple users / requests will take a lot of time