To keep yourself safe from any potential threat actor at any point in the future, and to prevent yourself from being manipulated psychologically via advertisements.

Imagine a carefree Instagram user, whose personal photos have been leaked. Can they instantly become free of stalking, impersonation attacks and cyberbullying? No. They will suffer a lot of mental harassment due to it, alongside other devastating effects.

Let us imagine, now, a privacy conscious person. They know what they post, where they post and how they post (their online persona). Can information be leaked on them, or them being impersonated, and all this linked to their real life identity beyond what they decided to show the world? No.

Therefore, privacy levels allow you to bind and limit the scope of what the world sees about you. Being uncaring of privacy means you become an open book vulnerable to all kinds of things at any point in future.

I have been a victim of impersonation and cyberbullying attacks as a privacy advocate in the past couple years. And yet, nothing fazes me because nobody can doxx me as long as I control the scope of data the world sees about me. That way, I can defend myself, and if they leave themself vulnerable, I can go on the offensive however I like.

As for advertisements, you can be data tracked and manipulated into buying useless and unneeded stuff on Amazon, eBay, your supermarket store nearby and so on. This is psychological stuff, for which ad makers and these shopping portals have hired neuroscience experts to play with human minds on a mass scale, using simple elements like vibrant colours, bold big text, neon signboards and such tricks.

To prove this psychological manipulation to yourself, just try changing display colour on your smartphone to grayscale, and watch the phone become too boring instantly. You will be surprised.

My threat model guide: https://lemmy.ml/post/34223