That’s a very good question. The short answer is yes! You should always carry a spare magazine. The magazine is what feeds your weapon and is a common point of failure. If you are clearing a malfunction and you release your magazine, you should insert a new one. The released magazine could be your point of failure and you don’t want to put it back in your weapon.
I am relatively new to firearms and just started carrying a handgun for self-defense. During the class I took to get my permit I was told I needed to always carry a spare magazine. What confused me was that in that same class the instructor also told us that most gunfights are over in 3 rounds or less. So if I am carrying a Glock 26 which has a 10 round magazine does it make sense to bother carrying an extra mag?