But it ignores them equally throughout the year… It’s not an exact measure, but that doesn’t mean it’s not a useful metric. So long as the measurement is unbiased and captures enough of the traffic, it can still be useful.

The troughs in that graph are all during prime US school/college vacation times: Summer, Winter, and Spring breaks. And then magnitude of the fall corresponds to how long the breaks typically are. To me, that makes a lot of sense.