Even if I use an addon that deletes my cookies from Google every time I close the tab?
For example: You're at your friends house with the Facebook app, your friend clicks on an ad for socks. The next day you start seeing ads for socks despite not having any sock-related activity. This is just an oversimplified example.
There are a myriad number of tracking vectors they can use. (And to be fair, so can any of the other companies out there tracking everything.)
Deleting cookies is certainly a start, but it's not sufficient to maintain privacy.
We used it mostly to try and track malicious users (banned users, etc.) but it's still tracking nonetheless. It was reasonably effective.
I'm sure a company of Google's size and sophistication has many more advanced methods than we did.