What is the purpose of the 5th precept?
The 5th precept is not about addictions. It is referring to taking substances that lead to heedlessness, whether you get addicted to it or not.

Love/sexual misbehavior is covered under the 3rd precept. Again, it's not about addiction.

The intoxication discussed under the 5th precept is chemically muddling your senses. Especially the mind. Ignorance plays a role in wanting to do that. Once you are intoxicated, it leads to more ignorant acts.