Thread protectors keep delicate pipe threads intact and unbent. However, these pluglike things that fit inside or around pipes (depending on where the threads are) aren't the only things that can be used as protection. A pipe cap may also suffice in some circumstances. Whether a cap would be a better choice than a protector really depends on the specific situation.
Fast Application and Removal
A pipe cap is going to offer the fastest application and removal for the most part when dealing with just a few pipes. But if you have a lot of threads that need to be covered, thread protectors can be applied with a machine, so that will save you time because you'll be able to go do something else and likely be faster overall. So if you have only one or two pipes, say, that you have to protect, pipe caps may work as fast thread protectors. Once you start getting into bulk amounts, though, you'll want thread protectors and equipment to apply them automatically.
Stable Protection for the Actual Threads
A thread protector doesn't necessarily have threads itself that intertwine with the threads in the pipe; you're more likely to see smooth sides. However, the protector should fit snugly enough inside the pipes that it forms a stable barrier covering the threads. That prevents anything that gets inside the pipe, such as a rock, from denting the threads. A pipe cap will, of course, cover the opening to the pipe and form a barrier. But if the cap is at all loose, even a little bit, sand can work its way under the cap, as can liquids that might dry on the threads and make them harder to use until you can clean out the pipe.
Internal and External Threads
One thing a cap can't do to protect threads is ensure external threads stay in good shape. You could find pipe caps long enough to cover external threads, but the cap material might not be strong enough to prevent damage if the pipe is jostled or if something collides with the pipe. For external threads, you would need a thread protector made to fit around the pipe in a specific size. These thread protectors can be metal or plastic, just like internal-thread protectors can be, but be aware that the fact that they are external and receive no protection themselves means they have to be extra strong. Otherwise, they could crack or suffer dents, which in turn could affect the pipe and threads.
If you often transport pipes and other items with threads that are similar in size, you may want to have metal thread caps in those sizes made ahead of time so that you always have a supply handy. Pipe caps can work for other items with internal threads where you don't have a supply of protectors.