It is usual to attach the hook to a line of slightly weaker than that attached to the reel, so that in the event of the line breaking either due to an unexpectedly large fish, or by being caught on an obstruction either in our out of the water, only the short length of line attached to the fish will be lost, resulting in less tackle lost or left attached to a fish.
You will see lines for sale described as for ‘hook lengths’ but in practice any line that is suitable as reel line will be suitable for hook lengths. In general a hook length of .5 lb (.23 kg) to 1lb (.45 kg) lighter than the reel line will be suitable, so the lines you buy for your reels will determine the additional lines you need to buy for hook lengths.