Identifying spam does not require building a profile of your browsing and communication habits, nor does it require information of what videos you watch on YouTube. If I put a task on my schedule to buy va from the nearby pharmacy, would that mean that I'd like to receive spam about it?

Habits is what we are. Habits make us predictable. There are habits that you don't know you have or just refuse to accept that you have them. That's why Google's algorithms know more about you than you do.