[danmaz74]

Tracking is the only way for Google to get traffic data; you could still not find that reason enough to get tracked, but it's not "zero benefit" for users.

[Aoyagi]

I'm quite sure they, and other map providers who have live traffic info (including HERE), get the data primarily from infrastructure managers.

[danmaz74]

I don't know if anything changed recently, but I'm sure that at least until a couple years ago they got the traffic info from their own app.