Greetings @anon87969570,
I think this is a great question- and one that I had to stop and think for a bit before I found an answer! I always appreciate when there’s a question that makes me stop and reflect 
Baneful Magick
While many witches follow codes of ethics (and there are many different ones out there- from the Wiccan rede, to the concept of karma, to personal ethic systems, etc.) that warn against the use of harmful magick, there are also witches who choose to not follow these codes of ethics and use various forms of harmful magick in their practices.
→ Related discussion: What is your opinion on Black/Dark Magick?
Black/Baneful Magick can be defined in many different ways- for the sake of the discussion here, I’ll use the term “Baneful Magick” to cover any magick cast with the intent of causing harm. This can include hexes, curses, revengeful spellwork, etc.
In general, if someone is casting with the intent to harm someone- they are casting Baneful Magick.
(I won’t go into “grey” magick or the discussion of defining what is “good” and what is “bad”- there is a lot of great talk about those conversations in the other Baneful Magick threads!)
→ Related discussion: Traditional/Modern Black Magick - What is Black Magick?
While it is true that some people do not believe in Baneful Magick, if you do believe that it is possible to direct magick to cause harm, I think it is natural to feel the urge to protect yourself from it.
Unintentional Curses
While Baneful Magick is intentional, some witches also use protection spells to protect themselves from unintentional negative energy. There’s a great discussion about the unintentional curses (even from things like compliments!)- you can check it out in the Nazar Evil Eye thread.
Non-Human Negativity
Some also feel the urge to protect from non-human “attacks”. Life is full of crazy things after all! Some feel the need to protect from all kinds of negative things or potential disasters that come from non-human sources such as: evil spirits, natural disasters, bad luck, general misfortune, etc.
While many spells, jars, and pouches have a specific target (protect from lack of money, defense against an ex-lover, protection from bad weather, etc.) I’ve seen the most popular defense spells tend to be against bad luck in general.
I hope that helped to give you a better idea about some reasons why protective magick is so popular! I’m sure others will have some wonderful suggestions for you as well.
Blessed be! 